HigginsI've not used a Thermarest as I'm too cheap.
But I've slept on my side on both a wood tent platform and concrete with a Klymit v2 and been very comfortable. If you over-inflate it won't be too comfortable.
My son who is in the Scouts has now decided he wants to use my v2. He was using my old blue foam. That's ok as I picked up the Massdrop EE quilt a few months ago and need the insulated version.
DismalDavei also own one and i've found the thurmarest to be quite a bit more comfortable also you can find them on ebay or at garage sales for quite cheep