I used to have the AP armor and helmet and i would mix and match my helmets. Ive had tons of ata helmets as well. Personally i think my AP armor was too slim. Im really skinny as it is btw. If i were to go again at another set, it would be ATA. Like Verne said. It has nicer arms. Plus i like the shoulder bells since they arent so skinny or rotj style. AP has a bluish hint in the plastic. ATA has a nice color to it.
All in all. You should look at other peoples armor and see which one suits you the best.