Well, of course it comes down to which games you want to play and which system you think has the most potential. X-Box obviously has the most potential hardware wise because its more powerful, but Microsoft is a newbie to the market and so they have the highest risk of failure. PS2 is a proven platform with at least 6 or 7 great games available for it already but the hardware is starting to show its age a bit. If you want a guarentee, just buy both

... But If you only want to buy one, just make a decision based on what games are out already and what games will be coming out in the short-term (next 3-6 months).
Don't forget if you want to go online, X-Box already has broadband support, but PS2 needs an upgrade. PS2 needs an upgrade for the hard drive too but it isn't a necessary upgrade. X-Box can also run as a hard drive based audio jukebox, but you need to rip the music off of the CDs yourself.
Hardware wise alone, X-Box wins hands down, but you can't ignore the PS2's existing library, backwards compatibility with PSX and Sony's current dominance in the market (although Sony was the newbie last generation, so it is totally possible for Microsoft to take the lead in the console market this generation).