No, this has nothing to do with sales.
I think that Gears of War and Halo, for better or for worse, have become synonymous with the image of the Xbox 360, and were for a time treated as rival franchises by many people. Now, from a sales perspective we know that's sort of silly, but in terms of game quality it might not be.
Both games appeal in different ways to different people for different reasons.
So, which do you like better: Gears or Halo?
Edit:
Uh... I guess I'll count all the opinions, too. I'll even keep track if you end up changing your mind.













“If you want to build a ship, don't drum up people together to collect wood and don't assign them tasks and work, but rather teach them to long for the endless immensity of the sea” Antoine de St-Exupery