I think people are missing the real discussion of this thread.
It's not about which will sell more, it's about which will sell more if you deduct the fanboys from it, only leaving in the real gamers, gamers who really care about gameplay, about the game.
I could have predicted Wii Play would sell more than Legend of Zelda, but how many of these people are actual gamers?
That's why I think Super Smash Bros. Brawl will sell more if you look it that way. You see, Nintendo players are not ********. If a game sucks, they won't buy it. How many of us Nintendo fans have ever bought Luigi's Mansion? I didn't, I only got it as a present. I never bought New Super Mario Bros. simply because it looked too much like just another platformer with Mario. I didn't play half of all the Mario Sport games because I'm not interested in sports game.
I am interested in every Star Fox game, simply because of my hopes of getting a good and decent Star Fox game. I loved the first Kingdom Hearts, not because of Disney or Square, but because I loved how the game was played. For that same reason I bought Kingdom Hearts 2. I didn't really care about the story, even though I did want to know how the story concluded (or not).
Heck, you can say all you want about the Grand Theft Auto series, I have had loads of fun for a while, doing drive-by shootings and running over some civilians. But I do have to recognize it gets kind stale after a while. But hey, every game does.
But really, what makes Halo 3 better than all these other games of that same genre? I think Gears of War deserves more credits, since it really pushed the bar for shooters. I haven't played it yet, but what I've seen so far shocks me. I even dare to say Halo 3 doesn't come near games like Bioshock. It just can't.
Sure, the majority of gamers don't like to play "kiddie" Nintendo games, but the same people mostly bought the console. The very same people just follow the trend, they follow the mainstream. Would that make them "real" gamers? Maybe, maybe not. But mostly they just buy games to impress their friends. That's not what a real gamer is all about.
Super Smash Bros. Brawl won't be the ultimate Nintendo game for the Wii. There will be loads of other games that will surpass this game. I'm pretty sure of it. But we do know that this game will be worth buying, it will be worth every single penny we spend on this game. I just can't say the same about Halo 3.