Bivy: Momentum 50 or SevenD?

I use M50 in a lot of the bivys I sell, and I have found it to be a great UL fabric. Weighs only ~.69oz sq/yd and has a very nice DWR finish.
I have never used SevenD so I can't speak for it, but I would definitely recommend M50.

Jace, either will work.

They're pretty much the same fabric (sub 10D and sub 1oz/sqyd). They both are calendared and have a quality DWR treatment. A lot of MYOG people have used and been more than satisfied with their M50 bivies. A lot of people also really like their titaniumgoat bivies which use SevenD.

You can't really go wrong with either. For the longest time they were also both roughly the same price. Now that M50 has been reduced to $13/yd I would suggest it just from a value perspective (save you $20-30 on your project which is enough to buy an extra yd or cuben to make a pack with!).

If you want to save even more money, the ti-goat KG-8D is even cheaper and still has a quality DWR treatment that should suit you well. The only problem is it's kind of a hideously bright green and they haven't released the new colors just yet (hopefully only a few weeks away).

I don't think I helped you at all in actually picking a fabric, but at the very least it should be nice to know that you pretty much can't make a "wrong" choice here! Good luck and post pics of the end result!