The Synmat 12 is rated to -4F, while the Megamat is -54F. I would think that -4 would be enough for most campers.
For some reason my Megamat has never done any self-inflation whatsoever. I've always inflated it by hand. The Synmat 12 and the Megamat inflate in about the same time. I recommend the smaller yellow inflation bag from Exped:
http://www.exped.com/usa/en/product-category/mats/schnozzel-pumpbag-ul-m I found the larger Schnozzel very unwieldy and could only get it 1/3 full.
I own both of these mattresses. I use the Megamat at home but prefer the Synmat 12 for camping because:
--It is very close to the Megamat in comfort.
--It takes up FAR less space when packed.
--Deflating it and rolling it up is MUCH faster. You really have to wrassle with the Megamat to get the air out. You have to roll, unroll, and reroll it about three times. The Synmat deflates easily in a few seconds.
Although you get a bit of extra comfort with the Megamat, to my way of thinking it's not enough to offset the considerable extra weight, packed size, and difficulty of deflating and rolling it up.