It depends (ps im torn between them too)
Do you like a polished versoin of a classic or a total twist on genre.
Basicly you know your gonna get a great game with gears 2, but it probabaly won't be very original, whereas halo wars is a complete twist which could be really amazing or really bad.
So with Halo wars its a gamble that could have a really big payoff and Gears 2 will definetly be great, but will it be amazing.
It depends on what risk you wanna take and what you personally like