Mostly depends on what kind of games you will be playing as well as if you will use it for a Hi-Def movie player right away. I've heard that Microsoft is in "preliminary" talks about whether to sell a Blu-Ray add-on for the 360.
Rock Band and Guitar Hero are made for both series so that is sort of a wash.
I play mostly sports games and here is what I've heard about those:
NCAA and Madden play better and smoother on the 360 for now. I don't know much about basketball games on either one. MLB The Show is without a doubt the best baseball game available and it is exclusive to the PS3.
I don't know if you have a home theater set-up but if you do, one downside to the PS3 is that it operates on bluetooth, so a regular universal remote (like the Harmony remote that I use to control my TV, Surround Sound, Cable Box, and 360) won't work on it.
I have a 360 and have been pleased with it. At the time I bought the 360, the PS3 had just come out and didn't have near the selection of the 360. (I still don't think it has caught up.) It was also 200 dollars more and wasn't worth it to me at the time.