I've had both and come to the conclusion that Ps3 is the best for the average gamer. It's free online, better graphics, less crap to buy/upkeep, no maintenance issues and of course Blu Ray.
If you are all into online gaming and clans and spend your life playing video games, then the extra money might be worth it for XBOX.
The argument that ps3 is more expensive is only made by people who don't have either system. By the time you upgrade your XBOX and play online for a few months it costs way more. Over the course of 2-3 years, its ridiculously more. Even little things like batteries in the controller.
If you want to game on a Microsoft system, get a PC.