The only experience that I have with Epic is the outer fabric of my original Patagonia Zephur jacket. The fabric is thin, windproof, and seems to wet through fairly quickly. I have never been nearly as impressed with the Epic fabric as with the eVENT fabric.
I know that Feathered Friends generally recommends Epic Fabric for outer material of their sleeping bags and garments for general use. It is lighter and windproof and water resistant. I can not comment about Epic outer sleeping bags having the condensation and frost problems that have been raised about Epic bivys.
Feathered Friends normally recommends the eVENT outer Fabric for their sleeping bags and garments for people that want the most durable fabric and most weather resistance. From my experience, the fabric is quite durable and wind proof (perhaps a little noisy). I have never experienced a water, condensation, or frost issue with my Feathered Friends eVENT covered sleeping bags or garments. Both the Epic and the eVENT fabrics will probably be more durable and down proof than the Quantum.
Be aware that though the 2 layer eVENT fabric that Feathered Friends uses for their sleeping bags and garments is durable, windproof, and water proof, they have not seam sealed the sewing needle holes. Therefore the eVENT Feathered Friends sleeping bags and garments can only be considered water resistant.
Backpacker magazine did review an eVENT Feathered Friends Lark Sleeping Bag which is online. The reviewer had left the sleeping bag partially out in the rain. They found that the rain water had pooled on the eVENT fabric and the down remained dry. You may wish to run a search on the Backpacker Magazine website to find the article.
Feathered Friends has been making eVENT sleeping bags and garments probably longer than anyone else and probably has more experience with the 2 layer eVENT fabric than any other manufacturer. I had Feathered Friends make up my original Lark sleeping bag in 1999 in what was then called PTFE Lite. At the time, BHA Technologies (designer and manufacturer of eVENT) had yet to develop the trademark of eVENT and Feathered Friends had been looking for a fabric as a replacement for Gore Dryloft and Gortex. Feathered Friends is without a dought one of the first companies to be licensed by BHA Technologies to use their 2 layer PTFE material (eVENT) for the making of down sleeping bags and garments.
eVENT can be overwhelmed by perspiration however, but for over 5 years it has been the best product available as a waterproof/breathable hardshell material. Unfortunately not enough of the big players in the industry use it. To show how much I believe in this material I have the following items in Feathered Friends 800 down, 2 layer eVENT outer fabric: Lark Sleeping Bag, Vireo Sleeping Bag, Hyperion Jacket, and Volant Hood. Additionally, I have the following 3 layer, seam sealed eVENT products: Lowe Alpine Elite Jacket, Lowe Alpine Evolution Pants, Integral Designs Rain Jacket, Integral Designs low gaiters. I will be purchasing either the Integral Designs or another makers eVENT Pants later in the year. eVENT has all but replaced all my Goretex Products.
Unfortunately GE who recently purchased BHA Technologies and eVENT has made a decision that due to the lack of flame retardancy eVENT fabric would longer be licensed for the purposes of making eVENT tents. As a result you will find discussions regarding this and Integral Designs eVENT tents on forums here on Backpacking Light. With a lot of reluctance I just purchased one of the few remaining eVENT Integral Designs MK1 tents from Pro Mountain Sports. I had intended to purchase the tent later in the year.
As to the durability of the Feathered Friends eVENT outer fabric for their sleeping bags and garments, my 5 1/2 year old Feathered Friends Lark Sleeping Bag is in great condition.