DSi is better for portability unless you did what I did when I went on vacation; getting a small tv and an AC to DC converter, then leaning back in the front seat with the tv in the back seat. Plug n' play.
Its DSiWare channel lacks good games except for Flipnote Studio.
Xbox 360 is good if you don't mind paying for wi-fi for XBox Live, which I've heard is very fun. Also, the XBox has many popular series such as Elder Scrolls, Halo(eek), and Call of Duty(also eek). Another thing: The graphics are amazing.
Ps3 has free wi-fi and also has many popular series such as Ratchet and clank(yaya) and much more. It also has Call of Duty and Some Elder Scroll games.
The graphics are good.
Wii is better for its wide variety of family oriented games and its wi-fi, but it lacks a large amount of good games. Graphics are baldsjdfk.