I see them both hitting around 10-12 million with Brawl edging out Galaxy. 3D Mario wont have the same pull as NSMB in Japan so I think it will fall short of those kind of numbers. Smash has always outsold 3D Mario in Japan and I think it will do it again but with both selling a lot more than their predecessors.

Sunshine was a poor game in the series
Melee was a good game in the series

Galaxy looks to be an excellent game in the series
Brawl looks to be a good game in the series

Galaxy will dominate brawl in sales.

Its a good question...

...and I would have to agree with ioi.

I think the multiplayer/online element in Brawl will give it awesome legs.

Galaxy will end up being the ultimate Wii "casual game"

Brawl will end up being the ultimate Wii "hardcore game"

(to be honest, Brawl doesn't appeal to me *that* much...) 

...

Galaxy should win this year easily, due to the earlier release (and Brawl not in Europe till next year). But long term... who knows?

Also going to be interesting to see how they both score - and how different sites review them both. Could they both get close to perfect scores?

Will sites like 1UP and Joystiq like Brawl better, due to multiplayer, online & conventional controls? 

When did main Mario titles start getting labeled "casual"?

main Mario titles were always casual, they're extremely easy to pick up and play. These days people think casual means void of depth and watered down, but that's hardly the case. Mario is easy to pick up and play but fun for anyone, that's not to say a casual gamer can master the game, it's just that Mario has a VERY broad appeal. Have you ever met a sane person that didn't know Mario? I can't say that I have...he's popular with all kinds of crowds.

My guess is Brawl will win. Main reason: replayability. Replayable games are keepers.

Many people will get SMG, but a portion of them will get 2nd hands.

Many people out of the Wii community are saying Wii is a casual console. But me and my friends had ran out of games to play together. The best get-together game is probably Mario Party 8, Wii Sports and Mario Kart 64. We are all waiting for Brawl and Kart and maybe Rayman 2.

On the contrary, they say Wii dun have enough hardcore games. Zelda, Paper Mario, Metriod, RE4, RE U/C, SMG, etc. There are so many of them. And I dun have enough time to finish them. And usually one copy of these games go around my group of friends. Comeon, we dun need 10 copies for 10 person.

Since most families will buy a party game each while many friends will share their hardcore games, I predict Brawl and Kart will win SMG hands down.

If you disagree, that may be because of my definition of games.
Casual:
1) Games that people play together more than they play by themselves, a.k.a. versus games.
2) Games that most people can learn and will pick up easily.
3) Games that you won't spend countless hours everyday for a week trying to complete them.
4) Games that you play and laugh at one another, enjoying the process again and again. Even if people watch without playing, it's pretty enjoyable.
5) Satisfaction is from winning other players.
6) E.g. Mario Party 8, Mario Kart 64 and Wii Sports. Brawl is here also.

Hardcore:
1) Games that people play by themselves, usually 1-player-only games.
2) Games that may be complicated for most people and has steeper learning curve.
3) Games that you will want to complete by pursuing it day after day until you complete.
4) And after completing, you probably won't touch it again. And your Wii party probably will be bored watching you completing a game by yourself.
5) Satisfaction is from completing the game.
6) E.g. Zelda and Metriod. I consider Paper Mario hardcore also because of the number of hours I spent trying to solve the clues and complete the numerous chapters. SMG is definitely hardcore.

Another point which is particular to Wii. Only versus games are getting online capabilities. Hardcore games aren't. So this could be another defining factor for casual or hardcore.

Anyway, I should be getting both of them, except that after I complete SMG, I'll lend to my friends. Brawl, keepers because I want people to play with me.

I am a casual gamer, I think =)

Mario I think will sell better, cause of casual appeal and the fact that they will regconize Mario very well and mostly very fondly.

It SHOULD sell better anyway, it definitely looks to be the much better game of the two. 

Bottom line, after this holiday season, ANYONE that was on the fence about getting a Wii will want one and anyone who was "waiting for the good games" to come out will want to pick a Wii up.

 i agree.

also, my gut says mario galaxy. 

Galaxy is going to appeal more to the current installed base and sell better than Brawl this year.

While Melee was slightly more popular than Sunshine on the Cube (6.58M vs. 5.91M), it was the long legs the game had that really helped Melee eek out the win. I believe that current Wii owners are still mostly casual and younger players. They will be more drawn to a platformer versus a fighting game. And SSB fans who are waiting for Brawl to get a Wii will have a heck of a time getting a system this holiday season and into early next year after the introduction of Wii Fit in January.

Long legs could really help Brawl win this 'fight' in the long term. But a real, high quality Mario game will give everything else on the Wii a run for it's money. And don't forget that the best selling game on the GameCube sees its sequel come out in February...