Can't be sure, but here's my $.03...
Some brakes, or compensators, are designed primarily to reduce felt recoil to the shooter, others- to minimize muzzle "flip" or rise, for faster follow up shots and target re-acquisition.
In looking at the two, it seems the top on, in directing the muzzle blast directly to the sides, would be to reduce felt recoil.
The lower one, while having the side ports, also has porting on the top side, which would aid in keeping the muzzle down.
I've been interested in these two as well...hopefully someone with more knowledge than us can chime in and correct me, or add to the info...