PS2, better games by far. The only good Xbox game out right now is Halo (which is coming for PC), and Morrowind (on Pc too) kinda if you love RPG's. Believe me, I have both Xbox and PS2. All the good Xbox games coming out Unreal Championship, Doom III, Splinter Cell, will or are also out on PC or PS2 as well.
Xbox's online plans are a little better, but I dont think it should be a big decider in choosing which system to buy.
In conclusion, a system is only as good as the games out for it, and PS2 clearly dominates in that area.