This is the silly little "ooh la-di-da" answer, but for me the Wii's significantly better than the GameCube because it has the exact same back catalogue, PLUS all the new spangly Wii games.
Even if there were only one worthwhile game for Wii, that's still left the GameCube redundant in every field but price.
But there's not! Metroid Prime 3 is classic. Smash Brawl. WiiFit's stellar if you've got the energy to keep up with it. New Super Mario Bros. The list goes on! And there's a splurge of phenomenal looking games coming out riiiiight about now... Skyward Sword, Donkey Kong Country Returns. Kirby's Epic Yarn looks marvellous, as does Sonic Colours.
My issue with the XBox 360 - and it's a fine console, mind - is that it seems to embody the "darker and edgier" side to gaming. It's populated by those games like Call of Duty, Splinter Cell, Grand Theft Auto... all these heavyweight juggernauts. And yes! They're well designed. Yes! They break very different ground, and do it well. No, they're not po-faced or dull - look at GTA, that's got the best humour-to-game ratio of most any game out there. But what I really love about the Wii is that it embodies escapism. It's that light, fluffy, imagination-reigns side, and I love being able to switch off that cynical part of my brain. Meanwhile, the XBox tends to inhabit the cynicism - the "all cobwebs and no ice cream" section. There are exceptions, sure - Viva Pinata for the XBox for instance is pure silliness, whereas Metroid Prime has barely a joke throughout the entire trilogy and it's one of my favourite games - but the Wii breathes the same air as all the fantastic games I grew up with. It's the only console which feels like it's still breathing the same air as Sonic and Lemmings and Theme Hospital - and indeed all of the old-school Nintendo games, too.
XBox games feel a little over-macho to me. Like putting a rocket launcher on a dragon. Sure, they're trying to be cool, and sure, they're definitely succeeding - dragons with rocket launchers are fantastic - but it feels more like hard work than play.