I own a 360, and have played the PS3.
One thing I must say about the PS3, it was way too overhyped by sony. The graphics are hardly comparable, Gears of war looks better than ALL of the PS3 launch titles, and there won't be many good titles for a while. I guess I will break it down in to categories.
Graphics/Processing power
In terms of benchmarkable processing power, the PS3 is more powerful, but in terms of programmable power, the xbox 360 owns the market by a landslide. 3 general purpose cores is much easier to program than 7 SPE's. Think of it this way, you are in a kitchen cooking a large meal for 15 people, you have seven different smaller dishes going at the same time, it is harder to cook 7 dishes than it is to cook 3 large dishes. Ok, I know that that is a pretty bad analogy, but it is kinda like what it is like. The xbox 360 also has a significant amount of graphics bandwidth available, something like 224GB/s while the PS3 only has 32GB/s or something around there.
In conclusion of this section, in terms of what the developers are able to program, the xbox 360 will be more powerful since it also has an excellent SDK (software development kit)
Online Play
Xbox live is now on its second console, they have an excellent system that many people love. Playstation network is brand new, and is not too great right now, but it probably will get better in the future.
Gameplay
The xbox 360 already has a slew of excellent games released, with plenty more good games coming, and there is rumble. There are also many cheap budget titles that are very fun and inexpensive. The PS3 just launched, and has a pretty bad lineup. The sixaxis features in games are pretty badly implemented, seems more of something they just tacked on. if you want an interactive experience, get a Wii, nintendo has done much more testing and bug squashing than sony has done with their technologies.
Availability
Xbox 360's are so easy to get, you can almost buy them at your cornerstore a block away from your house. PS3's and Wii's still have lineups when stock comes in, unless you want to turn to ebay.
All in all, a 360 is a much better buy, Gears of war is one of the best reasons IMO to get one, it is an amazing game, almost flawless (i have gotten a few graphical errors, but that is it).
But it is still your decision, but I highly recommend buying a 360 over a PS3, then you can also get a Wii on top of that and it is the same price (Wii+360) as a PS3 and a game.
Here are some comparisons:
Playstation 3 vs. XBOX 360 - A Game Developer's Perspective - hardCOREware.net Playstation 3 Vs XBox 360 | ITVidya.com GotFrag DPAD - DPAD Home News Story - End all arguments: PS3 vs 360