I have the Bibler hooped bivi, essentially the same as the Black Diamond hooped bivi. The hoop is a plastic rod that was permanently bent from being in the stuff sac. The purpose was to stiffen the opening so it would not be on your face. I pulled it out and never missed it.
The bigwall bivi has a waterproof tie in point so you can stay tied into the wall inside your bivi. Not a useful feature unless you have big walls in your future.
I generally agree with SJ. Out of a selection of expensive shelters I own, I use an inexpensive, light tarp for 95% of my alpine climbing, back country ski, and mountaineering trips in all seasons. FWIW, I consider a bivi sac as an emergency shelter for day trips or for alpine routes that do not have ledges large enough to set a tent on. Something I have only had to do a handfull of times in 20 years.
If you are set on a bivi sac, I would be happy to sell you my yellow Bibler (6' length) for $50. It is missing the plastic hoop which didn't work in the first place. Can be easily replaced with a stiff piece of wire or something.