I prefer double layer anything. I have been bitten by mosquitoes through single layer hammocks before (even through the single layer portions of certain double layer hammocks - WBBB there's a spot between the zipper and body at the top head end).
I just got my 1.9 DL.
I feel it "supports" you more than the single layer (I was able to lay in a 1.9 SL for an afternoon).
The SL I feel stretched a bit more (granted I'm 260 lbs., so that may have had something to do with it also).