I have the z-lite, but only for use with a prolite 4 for winter/cold climbing. I've never used the ridgerest, so I don't know how good they are. The z-lite/prolite have worked well for me down to 15 or 20 F, and that's a guess on temp but it should be close.
I think it depends on how cold you are/feel when you sleep and what you have been using. You may have an issue with going light at first since you've been using a heavy inflatable.