As far as Halo 3 goes, it can be very entertaining, but it really is a broken game, anyone who has played online as much as myself would understand, that whole **** where you and your opponet do the exact same thing, and you get the first hit, but for some reason your attack does nothing...its just so annoying...Halo 3 is broken, at least I know theres not going to be as many bugs in SSBB, Nintendo has a habit of making games that aren't broken.
For the most part, Halo 3, has the potential to be an awesome game, if the MANY bugs were fixed, but untill Bungie gets its act together and actually fixes the game, Smash will win hands down.
as far as Halo 2 vs SSBM goes, both were excellent games, Halo 2 was no where near as broken as halo 3, but still had some balancing issues, and SSBM was almost flawlessly made, except for its few bugs that rarely occur.
Both really are different genres, different games, they don't really compare on the same level, both have a high fan base and great multiplayer.
IMO the best way to go would be to play them both.