Downmat is heavier and warmer - more down in it than syn in the synmat! It also has an extra baffle to keep the down away from the two valves. The downmat also comes with the schnozzle dry bag pump, which does great double duty as a dry bag for your sleeping bag, etc. Using the schnozzle will keep the down nice and dry.
The weight difference is only a couple of ounces, so if you're sleeping cold now, it's probably worth the extra cost and weight. It would be a great thing to use the REI 20% coupon on!