That depends on what you want to do with it. Even then, ask 100 people, you’ll get 100 different answers and reasoning.
Technobuffalo did a 12-round comparison, that was pretty good. The link is to the finale, but you can view all the other videos too.
Both are great consoles, but each has its advantages. For example, even though PS3’s online service is free, Xbox’s is arguably better. Each console has certain exclusive titles, PS3 has blu-ray, etc.