You already probably got the game but I'ma throw my opinion in there anyways.
Reach was good compared to halo wars and odst which were both crap in my opinion.
1. Cod is WAY easier to move up in. It takes at most a couple of days to a week to prestege. In halo it looked like it takes a couple of days to get 1 or 2 lvls which to me personally is really boring.
2. Custimization: If you like custimizing more then just how your character looks go with Ops, you can put everything on your gun from the camo to the recticle color/shape in your red dot and everything in between.
But ya besides that its pretty much just preference when it comes to choosing