Of course I will test all factions that will make it into the game ;-) I am convinced that there will be more than two available... Every good RTS has at least three factions... And because the story plays before Halo 1 there are many possibilities for other factions. MANY!!!
Halo Wars has to be a "Console-RTS-Milestone" for many reasons. Why not use some creative freedom to lift the Halo-Universe to a higher level?
Back to the question. I'll play the campaign and in the multiplayer matches I play what makes the bigger fun (for me). But as always, it depends on...
;-)
I don't pay other people to pimp my gamercard ;-)