I have both consoles an bought about 5 games for XBOX and 40 for PS2. My main conclusion was
PS2
+ many games
+ free online service
+ downward compatible
+ plays DVD out of the box
+ compact
+ optical output (to connect to home cinema)
- needs memory card
XBOX
+ no memory card needed (but big HD is waste of space, only savegames and music is stored there)
- online service has a fee
- Few games
- Big (larger than a vcr)
- does not play DVD unless extra kit bought
- no optical output for 5.1 sound
As for graphic performance, I put them both side to side on a 40" plasma tv and there is hardly any difference. It mostly depends on how the games have been written. So instead of looking for the console, look at the games you want to play and if you need it as a DVD player too.
BTW, it seems that XBOX2 wil not play the regular XBOX games because the harddrive will no longer be included.