Which one should I buy, Synmat 7 DLX or...
Which one should I buy, Synmat 7 DLX or Exped SIM Comfort 7.5 DLX?
7.5 for car camping, the 7 for VERY cold conditions when carrying in your pack. If you do most of your camping above zero, consider the neoair as it weighs much less.
Get the Syn mat it is very comfy and can still car camp with it. However you cannot backpack with the comfort mat it would not be worth the pack space.