If you have only those 2 systems, I would imagine that the Gamecube version would be the one to get, but I am not yet aware of what is better. Wait for the game to be released, and read the reviews. That should give you the best info.

I really haven't had any hands on time obviously, and I might or might not, but I think you well know, I pretty much buy the XBOX version, but given that, The NGC is a superior system over the PS2. Should have quicker loading, maybe slightly sharper graphics, maybe smoother play. I can only base my opinion on what is known about multi console games out now, and thus far, games generally look better (same game title) on the NGC. Now maybe this won't be the case for future games though. Maybe look at reviews for Turok. That is a multi-console game that was released on the systems the same day, not allowing time to 'make it better.' There should be reviews soon. I would also tend to buy it for whatever system you plan to keep the longest - in case you were not planning on keeping one around forever, then why buy the game on that system, right!

Also I own a PS2 and have no problems with it, and an XBOX, I do plan on possibly buying a NGC, but not until next spring.

Well I have the XBOX version and it has very clean looking crisp graphics and multiplayer.
Also for anyone interested, the NGC version of the game has been postponed 2 to 3 weeks.

Hmm, well I only tried it with my Controller S, and I have no problems. The other 2 buttons are only for switching your target, and you can pretty much just smash em anyway, with out being to particular, so I don't see why the XBOX controller would be an issue. Also I like the analog stick placement better on the XBOX controller.