I'd go for Xbox myself, but that's only because I'd want Panzer Dragoon Orta more than I'd want Eternal Darkness. As it is though I have both systems so I'll get both games and be happy. 🙂
For games already released, then I'd go with Xbox. For future games, it really depends on how much you like First Party Nintendo and Sonic Team games. Since those are the best games coming out for the Cube in the near future. Star Fox, Mario Sunshine, Super Monkey Ball 2, PSO with GBA link for Nights, Eternal Darkness, etc. Xbox has many good games coming from the PC developer side if you don't want to or can't run the games on your PC. Also, you have good third party support for every genere except console RPG, but since you already have a PS2 that doesn't really matter. 🙂