Halo 1 is great fun, Halo 2 is greater fun because you can play as the covenant and run around with an energy sword (which basically makes you a god). I havent played GoW...but from what i've seen and read about it, i'm gonna have to vote GoW, it just seems to have more in depth gameplay than Halo.

Halo = shoot, throw grenades, decide which weapon kills which enemy the fastest, make suicide runs and come out alive

GoW = duck and cover, pray to god you can survive an ememy assault