I have the adventure bivy, the older version. I think they are the same size. I have never really used it, but it certainly seems tough, and is just big enough to maybe handle two people (at least me and one of my kids). It is also big enough that I can get access to my boots from inside, which seems critical if it really was an emergency and had to tend to wounds or changes layers etc. That seems less critical if you really have the shelter of a snow cave (I don't think of digging a snow cave in the NE mountains as all that realistic, so it depends where you are).
I would like to get the blizzard bag. Except I would have the overwhelming urge to want to pull it out and try it, which then means it could never really be restuffed. If you want one thing, this would probably be the best single choice.
I am also attracted to the bothy bags and emergency ski shelters, particularly when not alone. With a snow cave you don't need that, but having instant emergency shelter for the group would be really nice, particularly above treeline or if there was an injury. I bring a poncho tarp that more or less serves this function now.
For inside a snow cave though, you might as well save the extra weight of the blizzard bag and use the lighter bivy instead, and carry more real insulation, like a down vest or pants.
So no real help here, sorry.