I had both and sold my PS3 i find i was mostly playing Xbox as the online is a lot better and worth the 365 hkd membership per year, also game on xbox are a lot cheaper, just pick up the last splinter cell game (6 months old) new for 120 hkd.
Graphic are really similar as most of the game are made for both console and usually don't see much different. The ability to install game on xbox HDD is great to reduce loading time.
But if you plan on watching a lot of blueray then PS3 of course.