Well both bundles offer you a solid multiplayer experience, Gears and Halo are two of the most played online games.
Halo wars also has multiplayer and is a change of style, it is a solid RTS, good controls and entertaining, I would go for that bundle for that reason.
Still pick a copy of Gears up thou nice and chepa, other good multiplayer are COD4 and World at War, GTA is a lot of fun over live as is Battlefield.
Bioshock is the daddy of all shooters, great single player, still for me the best title released on the console to date.



