Initially the momentum's dwr will look just as good as Epic. But with a little dirt, sweat, abrasion, etc. the dwr on the Momentum will break down in as little as a day of abuse and require refreshing. At the least that means cleaning in a very rinsable cleaner like Akso Sportwash and reactivating by ironing or drying. Maybe reapply a good fluoropolymer DWR such as Akso's. Epic doesn't really wear out like dwr, and can be reactivated simply by washing. So Epic has better field maintainability than dwr treated nylon like Momentum. Obviously this is less important for a bivy than for clothing.
Epic works well in tents, bivys and gaitors, and winter gear where breathability and repellency outweigh true waterproofness. For rain wear it craps out pretty quickly even if you seal the seams because of capillary action through the porous fabric around pack straps, youy back panel, etc. DWR's behave similarly. If you need a real rain garment, neither Epic or a dwr only fabric can be relied upon day in, day out. That may also apply to bivy use without a tarp depending on your climate and bag insualtion. For a bivy any sort of hoop frame that will keep the fabric away from you as a taught panel will make both work better at keeping out water. Somewhere on this site are reports that Epic plugs up and stops breathing in bivies below freezing, which is outside my experience.
Epic will be easier to care for, but Momentum will be lighter by a couple ounces. The Momentum is available in broader color choices. Epic generates static and picks up dust readily.
So for a bivy, available Epic will be brightly colored, heavier, better long term water repellancy. The Momentum will be lighter, less repellant after a little wear and use, and comes in bright and neutral colors.