We have had 3x Downmats 7LW and currently on a Synmat 7LW. The Synmat is the better: it packs smaller, slightly lighter and is easier to both inflate and roll up. It’s also cheaper. Whilst it “R” rating is slightly lower in all practicality those numbers are irrelevant for normal biker type use unless you go to those insane euro winter rallies. I was recommended the Synmat by the team at Tamarack outdoors on the basis of its better all round performance and that it is more reliable….
… I have however found the Downmats to be unreliable in the longer term with either the membrane or whatever that stops the down from escaping failing, or the glue between the tubes failing so you end with one huge bubble. These two mats were about 6 years old when they failed though but the latest failure is on the wife’s Downmat - one big bubble, 3 years old but unused in 2020. It’s being replaced under the 5 year warranty but she is getting a Synmat as it’s replacement instead.
Despite all that they are probably the best addition you can make to your camping kit.