I love both Smash Bros. and Halo, so I'd like to give my unbiased opinion. Nah, I'd like instead to give actual figures, so then it will be completely unbiased.
Worldwide Figures
Halo: 6.43 mill.
Halo 2: 8.45 mill.
Halo 3: 6.56 mill.
360 sys: 16.48 mill.
Smash 64: 5.55 mill.
Smash Melee: 6.58 mill.
Smash Brawl: 0
Wii sys: 19.98 mill.
As you can see, Halo 3 has yet to out-sell Melee, but it's VERY close and will over time. Melee should instead be compared to Halo 2, as both are games that are phasing out. Halo 3 will most likely overtake Halo 2 over time as well. Any numbers on Brawl are just speculation. I do expect Brawl to sell astronomically, but out-selling Halo 3 is a big boast. However, the 360 has sold horribly in Japan (Japan: .56 mill, America: 10.52 mill), so Brawl already has a valuable trump-card. Plus the Wii has more systems overall out as well (360: 16.48 mill, Wii: 19.94), so it definitely has more thunder behind the engine, so to speak.
My personal prediction is that Brawl WILL outsell Halo 3 worldwide, but NOT even close in America.
Worldwide Figures
Halo: 6.43 mill.
Halo 2: 8.45 mill.
Halo 3: 6.56 mill.
360 sys: 16.48 mill.
Smash 64: 5.55 mill.
Smash Melee: 6.58 mill.
Smash Brawl: 0
Wii sys: 19.98 mill.
As you can see, Halo 3 has yet to out-sell Melee, but it's VERY close and will over time. Melee should instead be compared to Halo 2, as both are games that are phasing out. Halo 3 will most likely overtake Halo 2 over time as well. Any numbers on Brawl are just speculation. I do expect Brawl to sell astronomically, but out-selling Halo 3 is a big boast. However, the 360 has sold horribly in Japan (Japan: .56 mill, America: 10.52 mill), so Brawl already has a valuable trump-card. Plus the Wii has more systems overall out as well (360: 16.48 mill, Wii: 19.94), so it definitely has more thunder behind the engine, so to speak.
My personal prediction is that Brawl WILL outsell Halo 3 worldwide, but NOT even close in America.