So I've used this pad in just about all conditions so far. From the back of my subaru to mount Rainier with a little desert action too. This pad really is all season. I use the neoair xlite for going light in the summers too. I've heard that the Xtherm get very warm in warmer climates. I think in terms of durability, with the All season is a little thicker "feeling". What it can come down to is the shape! I have both pads in REGULAR, but with the mummy shape and idea of winter camping, going with the large would be better off.
So if you're looking for a do it all pad: All season. Winter specific, Xtherm.
Both are ultralight, ultra compact. I did end up buying a pumpsack to reduce condensation.