Given that despite waiting almost 18 months after starting development before officially announcing Halo 3 (and here I mean Bungie's announcement and their trailer, not Bill Gates' premature pronouncements) that announcement STILL came earlier than the Halo Wars announcement, I see no reason to expect Halo Wars to be released first.
In short, Halo 3 had been in development for a year and a half when it was announced, and it was announced before Halo Wars. Halo Wars has supposedly been in development for about a year when it was announced, and it was announced after Halo 3 was.
So unless for some reason Ensemble is able to churn out Halo Wars in 6-12 months or so LESS than Bungie needs to do Halo 3, it is logical to assume we'll get Halo 3 first.