They both cater to casual gamers, but within their genres, I think Halo 3 will appeal to FPS fans, while Brawl probably won't cater to as many hardcore fighting game players as Halo would to hardcore FPS players. There's also the fact that most PC FPS players tend to stick to CS and Battlefield 2. Halo imo has been the best FPS series ever for console, but it doesn't match up to other FPS when played on the PC, simply because it was made for console play. UT and CS sucked balls on XBox compared to Halo 2. I'll stick to playing them with a mouse and keyboard and playing Halo with a gamepad. There really isn't anything "easier" or "harder" than something else, considering difficulty is based on your effectiveness versus your opponent's.
There's also the fact that FPS and Microsoft in general aren't as popular in Japan. Japan will be a huge market for Brawl, and probably one of the smallest markets for Halo 3. Have you ever played a FPS with somebody in Japan? They don't know how to use dual joysticks at all... Halo 2 outsold Melee in Europe and Australia. This is due to Halo 2's online play, and the Gamecube's lack of popularity.
Imo as a gamer, I chose to make my investment on the XB360. This is because of Halo 2, Gears of War, BioShock, Halo 3, and GTAIV. BioShock for the PC has problems with AVG and installation in general, until Dark Coder came on the scene. I'd prefer BioShock on the XB360 to save myself the hassle and be able to play an awesome game. I know for a fact that I would only play Brawl on the Wii, and the only thing else that would really appeal to me would be Metroid Prime 3 and the Virtual Console games. The only problem with VC is that I have all of the VC games that I would want to play.
Sure the Wii's recent sales are outperforming the XB360's (at least in the U.S. and Japan; I don't know about other regions' consoles sales), but newer items will have newer sales. Take into account the fact that the XB360 has been out for nearly two years, while the Wii is only coming up on its one year mark. Less people are going to by the XB360 because many consumers already own it. Sure the Wii will catch up, but I think Halo 3's going to have a bigger burst of sales in the U.S. than Brawl would.
Brawl's release date is closer to the holiday season than Halo 3's is, so that's also a factor.
Ultimately, for people who will want to play both games (like me), they're going to get both. Hell, I don't have a Wii right now, but I know I'll eventually buy it to play Brawl...and a router haha. A key point would be multiplayer. Sure, Brawl has online play, but anybody who isn't your friend is completely anonymous, and there's no ranking system. There's a lack of sense of knowing how well you compare to others, but I'm happy there's no ranking system. Rankings = cheaters. That's not to say there won't be cheaters, that there will be certainly less.
Back on topic, Halo 3 offers more than just simple multiplayer. Online co-op is something that the community has wanted for a long time (co-op speed runs anybody?), and the Forge offers something extra for those with creative minds. Sharable maps, not to mention good user-developed maps will be added to playlists, the ability to take special screen shots, and wide-scale team play. Let's face it, most people rather play with their friends against others than playing against each other. My biggest gripe with Brawl is the lack of VoIP. So...it's a mute player playing against another mute player. I can't even say something like, "Good game." No VoIP, so where's my **** keyboard? Oh wait, I can't use that either, shucks.
Halo 3's the end of the trilogy and actually has a plot, so those who actually have deep knowledge of the story besides that **** offered in Halo 2, well, this is the end after all. Bestsellers for Halo books say something right there.
This is all a very, very long opinionated post; sorry haha. My opinion? I think it's going to be **** close and can go either way. I'm going to lean over to Halo 2 though, because these two games will depend on multiplayer. I see Halo 3 offering more multiplayer content than Brawl is atm.
On another note, as a reply to some of the posts I've seen in this thread... For those people who think Halo 2 doesn't take as much skill (people won't directly say it but they're patronizing it), well, maybe you should experience the pro metagame of whatever you're comparing. Most of the time when I see comparisons about how deep a game is, such statements are based on observations from a spectator's view, and not onf one who actually has played to a high level. I don't think any modern competitive games are deeper than others, since how you play is completely up to you. If you can't do something to win, then you just suck, not because the game "isn't deep enough" for you. If it's too easy, then why don't you go play better people? Speculation shouldn't be represented as fact.