Gears is far superior to Halo 3 in my opinion. Its graphics, gritty realism, & unique gameplay get my vote any day. To be honest, the Halo series has always seemed far too toned down to be anything close to realistic. Admittedly Halo 3 was a bit darker & grittier, with more drama, & characters being killed off, etc. but it's still nowhere near as realistic or brutal as Gears.
Even on the multiplayer front Gears is far more interesting, straying away from the "shoot, die, respawn" scheme from almost every game around. You actually need to work together with your team & be careful to survive. The fact that, unlike in Halo 3, you can actually mutilate your opponents with chainsaws, shotguns & explosives is far more entertaining than watching your enemies just fall over when killed.
I personally think the only reason why Halo 3 has such a strong head in this poll is that many people have simply been whipped by the hype & don't want to accept the cold, harsh reality that the game did not live up to its massive expectations.