Well, everyones gonna Have there own opinion, But by the Sells and Rates, I Would have to say Halo3, BUT Then again I Like Gears or war better for its, Take cover Sysytem and Team Commands, You should look into them and Find out what the games all about and then you see what game matches your Needs. Because, if someone here like gears of war there gonna say thats better same if someone like halo3. there gonna say halo3 is better, So like i said better off looking into the Information for both of them and see what meets your Needs.
Eh used games...$5 isn't worth it in my opinion (especially now that gears is like $30).
Personally between the two Gears of War is the better game. Halo 3's multiplayer is just a rehashed (teh shiny!) version of Halo 2 with some new maps and The Forge (which gets old fast). Gears doesn't have as much to offer but it's more in-depth and less of a run and gun shooter. The only thing Halo 3 beats Gears in is the 4 player co-op, going through the campaign with three friends truly was sweet.
Though like others have suggest Call of Duty 4 blows them both out of the water (though honestly the single player felt rather generic). Customization is great, it's not as 'finnicky' as GoW or Halo 3. In gears of war the host has a major advantage and in Halo 3 it seems the damage detection is random. You can empty a clip into someone's face with an assault rifle and they won't die, yet one burst of their gun will kill you. It's not always that bad but it is very frustrating.
So in short here's how you should pick.
If you loved Halo and thought it was the greatest game ever made (meh) pick up Halo 3.
If cutting into someone with a chainsaw and frantic battles are your thing grab GoW.
If you want an intense team based squad shooter with a high level of customization grab CoD4.
