theyre not very easy to compare. theyre both shooters, but have different styles and themes. one is third person, the other is first. gears has chainsaw machine guns, but halo has bigass energy swords. honestly, it depends on if you like faster style first person style, or slower, more observant third person style more. i personally like both, but i prefer halo's online. gears has a better campaign (than 3-reach anyways), better storyline, longer, with more ways to kill people, better graphics, better characters, cooler premise, etc. but i like halo's style of gameplay more, and the online is more entertaining and broader. plus firefight beats horde. then again, gears 3 has beast. which is beast, to say the least. i think i would rather OWN a copy of halo: reach and three rather than gears 1, 2 and 3, but i would still rent gears to play through the storyline and shit.
so all in all, halo. especially the first 2, they beat gears all together, hands down, i think.