One major difference is that it isn't possible to overdose on marijuana, while lots of alcohol in a brief period of time can lead to alcohol poisoning, which can be fatal.
Marijuana and alcohol have both been shown to significantly reduce reaction time and motor coordination making many activities when drunk or high, dangerous. Alcohol is known to contribute to liver disease, although it has been suggested that a nightly glass of red wine can help keep your heart healthy. Similarly, marijuana is known to cause short-term cognitive impairment (trouble thinking clearly), but for certain medical conditions, such as glaucoma, marijuana possesses therapeutic properties.
The health impact of any drug depends on how it is used, who is using it, how much is used, and under what circumstances. Marijuana and alcohol are no exception, so comparing them directly is difficult � each possesses the potential for unique risks or benefits. One further thing I would say is that alcohol tends to affect people around the user (abuse, violent conduct, dependency etc) more so than marijuana appears to.
Providing both are used in moderation and their use doesn�t affect me, I don�t have a particular issue. I think though I would prefer my child (of a certain age of course) to have the odd glass of wine than the odd smoke. I think smoking, however moderate, over a long period of time causes long term damage whereas (I may well be wrong) moderate alcohol use allows the body to recover and can provide some health benefits as I said above.