This is a question I also find myself asking. Given the similar pricing, I'm strongly considering AP.
One objective factor would be the replacement cost of the rotors. Another would be the availability of pads.
Has anyone found other reasons to go for Brembo GT over AP?
BTW, a while ago, I saw some comment about an AP kit "searching for vacuum", with the symptom being that the brakes would sometimes be a bit intermittent. The post indicated that AP knew about the problem and was searching for a solution. I think it was an 8-piston caliper, but I can't find it now. Does anyone have any details on this?