It really depends what you are after. I prefer PS2s over XBOXs, because they are an allround better console for gaming, but if you want it for something specific, like playing shoot-em-ups online, then an XBOX may be better. There is plenty of things to take into account though. If you want to build up a library of games, then I would advise against the XBOX, as there is not near as many top quality games, and not as big a range as the PS2. If you want to play platform games, that is another issue, as there is not as many on the XBOX. Actually, the Gamecube is definately the superior console for platformers, but it is the weakest console for first person shooters, so that probably counts it out.
All in all, I would go for the PS2, as I said, it is just an allround better console IMO.