It is difficult, isn't it! I like the look of the Hunka, but the comments on the AlpKit web site suggest that it is small, too small even to allow full lofting of some down bags. I think the larger Hunka XL bag is due out soon, if not already.
I think I'm coming around to a 3+ season bag because, in very cold temperatures, I'm always going to be in a bivi bag, tent, or other shelter.