Need some help building a Arc Alpinist

That choice should depend on what you're using for a shelter, among other things.

My Arc Alpinist is identical to your specs. I went with Pertex Quantum for the shell, pairing it with a Momentum 90 bivy (MLD Soul) and a tarp. That's a pretty common setup around here I'd think. Searching keywords epic quantum nunatak turns up most of the rationale. Same debate can be found about bivies. My experience with Epic-shelled down: its poorer breathability results in more accumulated condensation over longer trips (more than a couple of nights) compared to Quantum or Momentum, and slower dry times. Epic may be okay for shorter exposures to wet conditions, where resistance to outside moisture trumps the ability to dissipate internal moisture. Or where durability is more important. (I like my Epic Teenaway slippers for these reasons). But overall Quantum gives me a better balance in a quilt: excellent breathability, moderate resistance to wetting out, quick drying and light weight.

In context, I prefer to use shelters which are matched to the conditions – i.e. an ample 9X7 tarp in wet weather, or a light double wall tent for wet weather near freezing or colder. The extra weight of Epic wouldn't deliver more performance for me.

Just for clarification I have an Oware 1.1 Cat Tarp and a TiGoat Ptarmigan bivy. Hope that helps.

If you call Nunatak, Tom will be happy to discuss what fabric will best suit your intended use. He's a great guy to work with.