The games, the internet quality, the community, the customer service, the updates, the arcade, the indie games, minecraft, and more!
All of this depends on the person, meaning it's only better to you.
From my perspective, I prefer the PS3 games, the live is the same to me, community is the same (though some say Xbox Live is better because it's more stable, I don't a difference) , customer service is the same, updates are the same (Why argue about updates? They both update often and work well), and I don't care for the arcade, indie games, nor Minecraft, so that doesn't matter for me.
If you want to argue facts, then get to know the specs, sale rate, and most other things about the consoles that isn't just opinionated.
In the end it all depends on what kind of games you like and whichever console is more convenient for you.
I pick the PS3 because of the free live, the controller doesn't require batteries and I don't have to go out and purchase a wired controller, blu-ray makes things better, the disk tray doesn't scratch my games, the exclusives are what I prefer (I admit there are some exclusives on the Xbox 360 that I like, but not enough to make me change my opinion), it has an extremely low failure rate, doesn't overheat often, doesn't make loud sounds while playing, is relatively cheap to purchase, and some other small things that don't deserve to be mentioned.
However, I do have an Xbox 360, and it's a nice system, but if I had to pick between the PS3 and Xbox 360 then my choice will still be the PS3.
The Xbox 360 has some great things; such as the online is more stable according to many people, the controllers are great if you have somewhat large hands but are bad because you have to constantly purchase batteries or go out and buy wired controllers, the exclusives are nice, the dashboard has many more things you can do on it, and that's about it for me.
