Since the PS3 is not released to the public yet it's hard to get a straight forward answer. The XBox people will tell you to buy the 360, and the Sony people will tell you the PS3 > 360. They say that without actually playing both systems so to me? Their opinions are worthless.
Wait until the PS3 comes out & play both systems. I'm sure you have friends that will own both, go over and play them & then buy the better one in your opinion.
Although I believe the PS3' potential won't be reached until 3 or 4 years after the release date. Something to keep in mind, perhaps.