hey're two different types of games. Halo 3 is a first person shooter, and Halo Wars is a real time strategy.
It all depends on which genre you prefer more.
They both have good developers behind it. Everyone knows about Bungie. Ensemble is a little bit lesser known than Bungie, but they are the same people that made the Age of Empire games, and in my opinion, Age of Empires 2 is one of the best RTS game out there.
I don't think Halo Wars is going to set the records that Halo 3 did. Halo 3 was just a phenomenal success. It's expected to sell over 10 million copies. That's a lot for a video game. Most companies would be happy to just sell a million.