Get the 25" size.
I originally purchased the 20" wide (standard size regular). I'm a side-sleeper, and I rolled off it or felt like I was about to roll off it all night long. Had a pretty miserable night, really. Sent it back and ordered the size large (25" wide). Those 5 inches made all the difference in the world. Slept like the dead. Didn't need any time to get used to it. It's 5 oz heavier and that's 5 oz I will happily accept since it's still a full pound lighter than my old SynMat.
The trick to it, since it's a real air mat, is to blow it up first thing when you make camp, allowing the air in it time to cool and condense. Then when you go to bed put a couple more puffs into it and it'll stay well inflated all night long.