It really, really depends on who you ask. I prefer the plant overwhelmingly, myself.
The way I break it down is that alcohol is basically giving the devil on your shoulder a megaphone. Tendency for erratic behavior increases with alcohol. Along with generally making for a better time than being sober, it also has a tendency to exacerbate mood changes which can be plainly dangerous for people with anger issues, and it universally makes people hornier to the point that it becomes a pre-occupation.
With marijuana, I just find I enjoy things more. I get most of the benefits of alcohol without getting the mad hornies, and there is not much of a dark side to the experience. Generally, anyway. I have never had a hangover or had to vomit from marijuana (even though I have definitely smoked too much on several occasions). In addition to all of that, unlike alcohol where you generally become uncharacteristically more sexually aggressive but it can also inhibit your sexual experience, I find that if I find myself in a sexual situation when high, it's... whew. Awesome.
Getting high is also a lot cheaper than getting drunk.
For me personally, I have yet to have a truly bad experience (including repercussions/later consequences) with marijuana. That's not to say that that can't happen and I think I have gone about it's usage very responsibly (where depending on my work situation, I'll stay clean for months on end). Moderation is the key to everything.