DPMS's "A3" upper is just a flattop upper. The difference is that the DPMS A3 receiver has a picatinny rail on top so you can attach accessories or other iron sights. The carry handle you see is a detachable carry handle (and also why the rifle costs $100 extra).
The A2 has the carry handle built into the receiver and is much less flexible.
I always recommend people go with the flattop. Even if you never use any kind of optic at all, you have a much wider selection of iron sights with a flattop. If you ever plan to mount an optic, the flattop is much more versatile.
Unless you just want the detachable carry handle, see if you can order the AP4 without the detachable carry handle. With the $100 you save, you can choose from several rear sights (folding, same-plane, etc.) that may suit you better. Even if you do want the detachable carry handle, you can probably find one at a better price.