This is what I think I know... The Stubby is a full-on race/freecarve construction. I'm pretty confident the AM is softer. The shape of the Stubby includes a blunt, short nose, and a small tail with a little kick and the corners rounded off. The Stubby's running lengths are about 5cm longer than traditional. Why? Well, why not. It works. The shape and profile of the nose is derived from current World Cup shapes, so they are decambered and have a gentle rise. This helps with smooth turn initiation, and I'm convinced it has a benefit for variable terrain too. The rounded tail helps there as well.
They also have longer sidecuts than usual for their overall length. I got turned on to Shred's idea of less is more here when I got a pair of hand-me-down shaped skis that were 170cm with a 15m radius, and they carved great. There's simply no need for any extra lumber, imo. The 171 Stubby has a 13.2m sidecut, which is about the same as the Prior 183. My 170 has a 14m sidecut which is the same as a Prior 187. Those are pretty racy numbers, so the boards handle higher speeds and GS turns well. AM's have shorter sidecut radii for slower freecarving and/or smaller turn size.
Longer sidecut and shorter length makes for shallower sidecut depth. This pays huge dividends when not carving. Shallow sidecut makes the board very easy to push and steer around, and also easy to break out of a carve.
So all that put together makes for a board that is very capable as an all-mtn ride, without giving up a lick of pure carving performance. I would say if you spend your time 60/40 on-groom/off-groom (or more on-groom), and you like GS turns, get a Stubby. If you spend your time 40/60 on-groom/off-groom (or less on-groom) and/or you live in an area that gets frequent powder, get an AM. If you're time is spent 50/50, well, flip a coin I guess. I can't comment on moguls because I'm not a big fan of riding moguls.