They are built on the same trees - so the fit should be the same for your horse, too.
For me - YMMV - the Gen X rides more open through the seat, the AO sorta puts me in place. I prefer the more open feel. I think the Gen X feels a bit wider through the twist, though marginally. Obviously the flaps are different as well.
Thanks for the information! So is the A/O a more forward flap? I think one of the reasons the Gen-X fits my horse is the fairly straight flap does not impinge on his shoulder. But I'm getting knee pain and I'm sure part is the seat is too small, but wondered if there were other differences as well in different styles.
Well, both models come in a plethora of flap configurations now - forward, long, forward long and regular - so you can pretty much get whatever fits you. The shape of the AO flap is just a bit different, a touch forward, perhaps, and they have the molded knee pad as well on some models.