I own and have used both the regular and the SL, both in the regular width, not the wide. The SL is am r-4.5; the regular is r-5 - and either one keeps me warm for April through November in Ohio/Kentucky/Indiana. In the regular width, the SL is an inch or so narrower, fully inflated, than the regular - take your regular-wide to the store with you, get them to inflate an SL wide, and lay it on top of the regular and you'll be able to gauge the difference in the newest model. Then lay them side by side, and spend some time on each to see whether you see any real difference. The important thing is how you feel on it, not how I tell you how I feel on it.
Having said that: I've found that, in practice, the slightly narrower width makes no practical difference to me. Both are equally comfortable, and I didn't have any trouble staying on the SL. One night at home, when I was totally, I did the the side-by-side test; I found that , on my side with my knees bent, my kneecap hung over the edge, but it didn't touch the floor. That might matter on a 10-degree night, but for 3-season camping, I decided to save, the half pound, and chose the SL.