It all depends on what you want.
PS2: More RPG's, backwards compatibility with PS1 games, generally more Japanese-oriented games than the XBOX.
XBOX: Pre-installed HDD, (supposively) better graphics, more American-oriented games produced by Microsoft.
Since you're getting a modded system, here are a few suggestions:
1) If you get a modded PS2 -
Invest in a large HDD and the HD Loader. That way you can store games on and access them from your HDD.
2) If you get a modded XBOX -
Make sure you get an Xecuter brand chip, preferrably Xecuter3. Also, invest on a 200 Gig HDD so you can store all your games on it.
XBOX doesn't need an HD Loader program, but it does need an illegal BIOS and Dashboard installed on the modchip (which I can not link you to on this site, sorry). Check out
http://www.xbox-scene.com for more info on everything XBOX. The HD Loader costs a pretty penny (last I saw it was at $60?), but it doesn't require you to search the internet to find an illegal BIOS and dashboard to flash your chip with.
If all else fails in your decision-making: Get both. They're both very amazing systems.