I use my PS3 for exclusives, blu-ray, and backwards-compatibility, so be smart and grab a 60 gig on eBay - that way you'll get the most fully capable system at a reduced price.
As the list of PS3 exclusive gets bigger (what is the number up to now, like 10?) it'll be more and more worth it. Multi-platform releases are usually better on the 360 anyway, for various reasons (better controller, DLC, online, etc.), but again the exclusives/blu ray/BC features make the PS3 a solid purchase, especially if you can get it on the cheap.