Years ago I used both the Quantum and the Endurance versions of the Marmot Helium bags and found that over a few days time the Endurance bag would lose a bit of loft during 3 season use in the Northeast. This was a concern due to the constant high humidity as the bag could not really dry out completely in the sun. I have and still use the Quantum version.
I think the other poster is correct that Endurance is a PU coating added to the fabric. Here is a conversation myself and others had with Feathered Friends about it ages ago… they seem to have since begun to use Endurance again.
http://cascadeclimbers.com/forum/ubbthreads.php/ubb/showflat/Number/765103
"Feathered Friends used Pertex Endurance for a number of years on our products, but the last time we had an Endurance garment in the store was 2004. We still have some of the fabric, and use it for bags in our rental fleet.
One of the advantages of Endurance (and especially the newer variant, Quantum Endurance) is light weight. I don't have exact specs on hand, but I estimate the weight of EQ to be a hair over 1 oz/yd. We have a couple new prototypes made up from EQ in the shop, and they are ridiculously light. Endurance does have pretty good water resistant properties, but we stopped using it for a variety of reasons. Chief among these are breathability and durability. Unlike ePTFE membranes (like eVent), Endurance is a polyurethane coating. This gives it less breathability than eVent. We haven't done any scientific testing on it, but our staff who have used both fabrics report that Endurance doesn't breathe as well as eVent. Then again, no laminated/coated WPB fabric does."
