Both systems have the potential to run games with anti-aliasing. That's not just for the ps3. Both systems handle the process differently, and to different results based on the game. But it's there for both.
To answer your question, op, it depends on what games you want to play. If you are going with one console, it should usually be largely based on what first-party games you want to play. That said, they are two vastly different consoles.
Xbox gives you a better online service, albeit one you have to pay for, and they seem to usually get better performance/visuals on third-party games.
PS3 has bluray, allowing you to watch hi-def movies, which is nice. They also have a larger list of exclusives. Though, I always found that bluray was less than optimal for gaming, being that it tends to effect things like load-times, and makes installing many games a requirement, which can take up a lot of space.