Having gotten cold on pads supposedly rated to 20*F (bought before they included R values in the specs), I got the Downmat. I definitely sleep cold and would like to be able to stay warm down to 10*-15*F. (I do most of my backpacking at high altitude, where snow can happen any month of the year and frozen ground can occur in late September-early October [larch season].)

If you won't be dealing with lower than mid-20s F or with frozen ground, you may be fine with the Synmat.