Look at it this way, Brawl=Halo 3 and Galaxy=BioShock
Both very great games, 1 game noted for competitve play, and another for just pure awesomeness playthrough.
Brawl is good because it has awesome replay value, considering there will be so many characters to master, and wi-fi makes the game undieable for a long long time.
Galaxy takes all of the mario platforms, no ALL platforms, and makes them look microscopic. But after playing through the game a couple times, getting the best scores, getting all the stars, finding all the hidden portals, and such and such, it will go old.
Galaxy>Brawl in short doses, Brawl>Galaxy in the long run.