Personally, i like them both. It mostly depends on what console you have, since Halo 3 is for Xbox 360, and SSBB is for Wii. I have never had the opportunity to play Super Smash Brothers Brawl, but i have Super Smash Brothers Melee for my Gamecube. Halo 3 have i only played when i have borrowed it home, or when i am at a friend's house. So, as said before, it also depends on what age the player is. If the player is 15+ he/she might not appreciate Super Smash Brothers Brawl that much. But, it also depends on what kind of person you are.
Halo 3 is more of an actiony, Shoot em' up game and most players on Halo are rather rude. SSBB is more user friendly and better for casual gamers. There is really no similarity between the 2, but still..