Having owned a number of bags with "waterproof breathable" shells over the years, I am a big fan of the most breathable bag fabric possible and using a bivy if you need more protection.
Previous shell fabrics include: Gortex, Gortex Dryloft, Epic, Pertex Endurance, and several proprietary fabrics from Sierra Designs etc. The problem I experienced was moisture buildup within the bag over time due to the dew point occurring within the bag. I have not tried eVent as a shell cover, but I would still prefer a very breathable shell and then an eVent bivy if I thought I needed the protection eVent offers.
I don't use a bivy very often but find that Pertex works well for me. Based on the results of Momentum,(not Momentum 50) that was tested for water resistance by Richard Nisley in the cuben fiber thread, it looks as good as some silnylon and is purported to be very breathable and is available for sale either as fabric for MYOG or in a completed bivy by a number of cottage gear makers.