"Being a video game developer (I develop for both, Playstation 3 and XBOX 360) people ask me almost daily which platform I think is better. These are my personal feelings, in no way does this reflect my employer.
Short answer: XBOX 360.
Long answer: Price, performance, visual quality, game selection and online support. I think the XBOX 360 wins in every category.
Price: This is obvious; the XBOX 360 core is only $299. The PS3 is around $499 for the 20GB version. It comes with a hard drive, but you don't need a hard drive to enjoy a lot of great games on the 360 so I think it's fair to compare both core systems.
Performance: On paper, the PS3 is more powerful. In reality, it's quite inferior to the 360. Without getting into too many details, the three general-purpose CPU's the xbox360 has are currently FAR easier to take advantage of than the SPU's on the PS3. I suspect a few years down the road some high budget, first party PS3 exclusive titles will come out that really take advantage of the SPU's and do things the XBOX 360 can't, but I don't think the console is worth buying based on this speculation (for some it will be though, we'll have to wait and see how these games turn out).
Graphics: The XBOX 360 is a clear winner. The GPU is more powerful. It has more powerful fillrate, and far more pixel and vertex processing horsepower. Part of the reason is their choice of memory, and architecture of pixel and vertex procesing. I can't get into details but the same vertex shader will run much slower on the PS3 than the XBOX 360. The 360 also has a clever new way rendering high definition anti aliased back buffers. To accomplish the same effect on PS3 is prohibitively expensive. For this reason I think many games will have no choice but to run in non-HD resolutions on the PS3 version, use a lower quality anti aliasing technique, or do back buffer upscaling. The end result in all cases is going to be noticeably worse image quality..."