This generation, I bought an Xbox, but I'm thinking I should have gotten a Gamecube. Xbox has more third party games, but most of the third party games I want are also on GCN (though some, like THPS, would be difficult with the GCN controller). There are more GCN exclusives I want than Xbox exclusives (for example: Wind Waker, Mario Sunshine, Mario Kart, Smash Bros., Metroid Prime). The only game I have on Xbox that's a must-have over anything on GCN is Knights of the Old Republic (when Fable comes out though, that could change).
With that sentiment in mind, I think I'll go with Nintendo's system next time around. I'm getting tired of all the stealth, war, and crime games, and I like Nintendo's philosophy on keeping games simple and fun. Depending on Nintendo's third party support, I may have to pick up Microsoft's NextBox to get my THPS fix though. As for Sony, I like their electronics (digital cameras and TVs especially), but I'm not a big Playstation fan, so I won't be buying their system.
Either way, I have a backlog of PS1 and DC games to play, plus more I want to get, so I probably won't get any new system until a year after it comes out. Then again, I'm a sucker for early adoption: I bought 32X, Saturn, Dreamcast, and N64 launch week. I guess I'll play it by ear, but I'm leaning toward Nintendo.