I own both a PS3 and an Xbox 360. I like my Xbox 360 more. I have never had the problems most people have with their XBOX's. I have had mine since 2006 and no problems yet. I generally get a better experience out of my xbox. Less load times, faster framerates, and I also tend to like the exclusives better. I also, like the online format better. I think it is easier to look at and easier to use. I also like the controller design better on the xbox. It feels nicer in my hand compared to a PS3 controller, but that may be due to the fact that I have large hands.
That is not to say I am a fan boy for the xbox. I think the PS3 is a much better system. It is more powerful and by all accounts should be faster, but it is not from all of the games I have played. I think this is due to developers not fully utilizing the hardware, though. Not the PS3. I love the fact that PS3 games come on blu-ray discs. Far superior to standard DVDs. I mainly use my PS3 as a blu-ray player, aside from the PS3 exclusives.
On PS3 I mainly only own the Sony published games. Like God of War, Metal Gear Solid 4, LittleBigPlanet, etc... I feel like they run perfect on the system because Sony knows what they are doing on their systems. Cross platform games however, I have always noticed the xbox performs just a slight bit better. Less load times, better framesrates, and graphics usually are better as well. There is also not a mandatory install on the XBOX as with most PS3 games.
The best game I own on both platforms that I can reference is Bioshock. On the xbox the game runs smooth with no loading screens in the levels, only between levels. On the PS3 as soon as I put the game in I had to wait 25 minutes for the game to install onto the HDD then once it is installed onto the disc for the game to run smoother there are load times between levels and during the levels. It's kind of a bummer. The only additional up on the PS3 version of Bioshock is that there are additional levels to play.
That is my two cents in on the two systems. Honestly, either system is a great choice. I would not part with either of mine. I think the Xbox is slightly better performance wise, but the PS3 definitely has advantages. I like the ease of connecting with my friends on Xbox Live and I also like the fast performance on the Xbox. If you're looking for a multimedia powerhouse go with the PS3. It has blu-ray capabilities and if you're just wanting to play sports titles and the occasional game it has all you need.