Ah thats a very good question. ill break it down into 2 sections of details
Halo 3:
A very wide played game online. If you desire the sci fi like action with aliens etc and are a fan of the halo 3 series then i recommend getting it. Halo 3 also allows you to do forge, which is a mode where you can delete objects and make your very own maps etc to your likening as well as play them. You also have the option to take screenshots in theater where you can review how you did in games, or forge and take screenshots or film clips. And heres the thing, in multiplayer your givin the option to choose what the color of your armor is, what armor you want and also the emblem you want along with it. Halo 3 is a great game that will keep you entertained for days or months with its online activity.
GOW:
Depending on the one you speak of GOW or GOW 2. I have not played these games for very long but from beating them bother i will say this. GOW involves creatures called the locust. you can think of them as being ugly or alien like but. If you want hardcore action that involves major battles and good amounts of blood i would get gears of war 2. but if you havnt played the first get it then get 2. because the game involves much more activity and graphics are increased greatly. This i my personal opinion but i believe GOW is longer than halo 3, thats what i believe and it gives you the thrill that you are seeking for will intense blood fights and a variety of weapons hoped that help
GT is liop1121 if you want to add me if you have any questions