As various legalisation models for cannabis are being proposed and implemented in different parts of the world different ways to consume the plant are also becoming more popular. The 3 main ways to use cannabis are smoking, vaporising and eating (though suppositories, topical applications and even vaginal insertions for women often used for medical applications of cannabis oil).

The primary difference that both smoking and vaporising has compared to eating is the route it takes through your body. While eating cannabis involves cannabinoids going through your digestive system and through your liver. Because of this eating cannabis takes between 40 minutes and 2 hours (depending on other factors) to exert psychoactive effects, which makes titrating doses very difficult.
In contrast to this smoking and vaporising delivers cannabis to the brain very quickly, with in seconds, as cannabinoids are absorbed through the lungs and then sent directly to the brain via the heart.
Eating cannabis may be preferable for treating serious illnesses, like cancer, because it allows larger doses to saturate the entire body, as opposed to concentrating doses in the brain, and suppositories are becoming more popular for a similar reason (with a superior absorption rate observed through suppositories compared to oral ingestion, along with other advantages which include lower psychoactive effects which is largely due to the fact that suppositories generally by pass the liver where Delta 9 THC is converted into the more psychoactive 11 Hydroxyl THC).
But while eating cannabis (or even better using suppositories) may be the more effective way to saturate the body with cannabinoids, which is desired for treating many conditions long term, it is not so ideal when using cannabis to treat symptoms, largely because the effects are so delayed and doses very hard to titrate as a result.

So for people who need immediate relief the delivery of cannabinoids through the lungs, experienced when smoking or vaporising, allows almost immediate relief as the cannabinoids reach the brain within seconds, as opposed to the 40 minutes to 2 hours that is often required when ingesting cannabis orally.
This allows doses to be titrated with tremendous precision and users are able to experience immediate relief for treating various symptoms. For recreational users these immediate effects are also very desirable as is the dose precision facilitated.

While smoking and vaporising are similar to each other in respect to the delivery method and the resulting advantages of providing immediate effects, there are some very significant advantages that vaporising has over smoking.

Probably the most obvious differences between smoking and vaporising is the health advantages of vaporising cannabis. Although smoking cannabis does not expose users to the same health risks as tobacco smoking it does still expose the lungs to smoke and inhaling combusted plant matter does involve the consumption of harmful chemicals, including recognised carcinogens. The largest study conducted on smoking cannabis has shown that not only does cannabis smoking not represent an increased risk of cancer, despite the presence of carcinogens, but the study suggested that smoking cannabis actually reduced the risk of developing head, neck and lung cancers with cannabis smokers less likely to develop cancer than even non smokers. The likely reason for these seemingly contradictory results is the cancer killing properties of cannabinoids like THC, that likely offsets and negates any carcinogenic effects of the smoke present.
http://www.scientificamerican.com/article/large-study-finds-no-link/
Despite the lack of established cancer risk from smoking cannabis, along with a failure to establish other health risks, the fact remains that cannabis smoke, particularly when smoking herbal forms of cannabis (as opposed to "dabbing" oils or similar extracts) does contain many potentially harmful chemicals.

The health advantages of vaporising cannabis, compared to smoking, is primarily the absence of these harmful chemicals as combustion is either avoided or drastically limited when cannabis is vaporised. This allows user to consume vaporised cannabinoids with out the harmful by products associated with smoke.
Both smoking and vaporising cannabis involves the inhalation of vaporised cannabinoids, but smokers are also inhaling a cocktail of potentially harmful chemicals in amongst the vaporised cannabinoids and while the presence of the cannabinoids does appear to offset some of the potential harm caused by the smoke the health advantages of consuming these cannabinoids without the inclusion of smoke is obvious.

In addition to the health advantages there is also other advantages that vaporising has over smoking.
Vaporising cannabis provides a far greater yield of both cannabinoids and the terpenes which have shown tremendous synergistic effects, as well as standalone benefits, including but not limited to anti inflammatory properties.
http://www.ncbi.nlm.nih.gov/pubmed/21749363
This increased yield is largely down to the fact that smoking cannabis actually destroys a significant percentage of the terpenes and cannabinoids as they are combusted, rather than vaporised, leaving the user to inhale a mixture of vaporised cannabinoids and harmful by products of combustion, where as vaporising essentially provides vaporised cannabinoids with little to no combustion involved. So rather than wasting cannabinoids as so many are combusted and lost, vaporised cannabinoids provides a significantly larger yield, making vaporising a far more efficient, and as a result more economical, method of consuming cannabis than the comparatively wasteful method of smoking.
http://www.canorml.org/healthfacts/Second-Study-Shows-Vaporizers-Drastically-Reduce-Toxins-in-Marijuana-Smoke
Studies on vaporising cannabis has shown the potential for vaporising to reverse conditions caused by smoking
http://www.ncbi.nlm.nih.gov/pubmed/20451365
With patients largely preferring natural cannabis to pharmaceutical drugs that are cannabis based (usually involving synthetic cannabinoids, rather than naturally occurring cannabinoids) and studies demonstrating a far greater efficacy from natural cannabinoids, as well as the superior efficacy associated with naturally occurring combinations of cannabinoids, which is known as the entourage effect, where a combination of naturally occurring cannabinoids appears to produce benefits far better than isolated cannabinoids, essentially demonstrating that cannabinoid combinations, as found naturally in cannabis, are greater than the sum of their parts.
http://www.tandfonline.com/doi/abs/10.1080/02791072.2013.805976
http://www.leafscience.com/2013/11/04/patients-find-cannabis-effective-cannabis-pharmaceuticals/

The advantages of vaporising cannabis, along with the medicinal value of natural cannabis in general, are so well established that vaporising cannabis is considered to be both a safe and healthy, as well as an effective, method of consuming the plant. With Israel appearing to lead the way both in regards to the use of natural cannabis medicines and in particular the applications of vaporising as the delivery method.
http://cms.herbalgram.org/herbalgram/issue97/hg97-featcannabis.html?ts=1384026817&signature=3715c09369e4572cc7264013cdce4393&ts=1454852928&signature=07094e28941b24a505c139202ffefeac
One of the more sophisticated vaporisers, the Volcano, is already approved in Canada and the EU as a medical device, despite that lack of approval in many EU countries for cannabis itself as a medicine and the continued prohibition in many European countries of cannabis for both recreational and medical use.
http://www.ccic.net/index.php?id=132,744,0,0,1,0

While standard vaporisers generally provide the basic benefits of vaporising and possess the advantages over smoking (primarily providing vaporised cannabinoids with out the harmful by products associated with combustion) the more sophisticated vaporisers allow for even more precision when it comes to cannabinoid consumption.
Because cannabinoids, and terpenes, vaporise at different temperatures the ability to control the exact temperature, a feature more common among the more sophisticated vaporisers, allow users to provide a more specific dose of certain cannabinoids with many patients reporting preferable effects from certain temperature ranges.

Though it cannot be denied that many cannabis users prefer the taste and effects of smoked cannabis to vaporised cannabis, though many users will make the exact opposite claim, generally speaking vaporising has numerous advantages over smoking while smoking has virtually no advantages over vaporising.
Vaporising cannabis is more healthy and more economical (at least in the in the long run) and provides far greater yields of both the cannabinoids and the terpenes present. The effects are different with many patients who use cannabis for medicinal purposes, as oppose to recreational use, reporting that they experience a more clear headed high from vaporising, where as smoking cannabis is described as more narcotic like in the experience it provides.
Innovations like dabbing cannabis oil, for example, do negate some of the potential harms associated with smoking (as the lack of plant material in cannabis oil equates to a lack of tar in the smoke and makes the experience more akin to vaporising, the issue of combusting, and hence losing, a large percentage of cannabinoids from the heat involved in smoking cannabis the most efficient way to consume cannabis with out wasting cannabinoids and with out consuming harmful chemicals is clearly vaporising and as a result vaporising cannabis is considered the optimal way to consume cannabis through the lungs, especially when used for medicinal applications. With the increased yield also making it preferable for many recreational users.