I own everything out curently. Between these two, I'd have to go with a Gamecube.

X-Box has a definate advantage with online games, but I play my online games on my PC, PC's definately rule the online department.
Sure, X-Box is the most powerful system, and yes it has Halo, but ok, what else?

The other few games I own for it, are either out, or coming out, to the Gamecube. Cube however, has for it that is NOT coming out to X-Box (or PS2)

Eternal Darkness
Metroid Prime
Zelda (in February)
Mario
Rogue Leader (old game, still good)
Bounty Hunter (don't think it's going to X-Box)

-Bounty Hunter I have't quite picked up yet either

X-Box does have it's share, such as Panzer Dragon, Mechassault, and Steel Batillion for example, but overall there is more for GC that I personaly want. That and X-Box is DRY for RPG games, and I love RPG's.

I personally would get the Cube if I had to choose, but I do like having both. I don't think X-Box will truly shine until next holiday season when live is up and running, and they can get more games out for it. (Steel Batillion II)