MPR all the way! I have two and they have given me some of my best scores in HFT, and I own or have owned all of the above guns mentioned. I also still own a GML'd AA 400 and a GML AA 410, and Chris is spot on with his comments about the thicker barrel and adjustable trigger. They also have a dedicated .177 pellet trough which helps when loading.
AA400 is very good but the MPR 400 is a superb all rounder.