Different but still great fun!
You can play as either the Covenant (Elite led /Brute led /Prophet led) or the UNSC (Captain Cutter / Professor Anders / Sergeant Forge) outside of the campaign. The game has been played for hours on end by me and it is a must buy for any Age of Empires fans as they are very similar to each other. The game though is highly repetitive and you will find yourself just building up forces then throwing them in one big mob at the enemy base again and again.
Overall, Halo wars is an underrated game in the halo franchise and is slated a lot for not being FPS, this I believe is highly unfair. Halo Wars though suffers from repetitiveness constantly, please take this in to account before purchasing.