I've got the Helium bag, so here's my $0.02:
1) you can't get much lighter than it, so I didn't find any pt in getting a 30 degree bag. If you think the Helium's too heavy, you'll prolly sleep w/ belay jacket & close friend
2. I wouldn't use it for winter volcanoes, but I think Cracked uses a similiar bag (the REI Sub-Kilo) for winter snow trips. He must run warmer than me.
3. Helium is big enough for me & my awe-inspriring chest.
4. duh. No mouth-breathing into your down bag. No wetting the bed. In the cold dry climes of Alaska and Himalaya you can dry out your clothes inside the bag, but I don't recommend such in the PNW.
5. Your call. I like extra space in my bag for hot water bottles during glacier climbs/ snow camping.
All that said, I use the Helium from April-October. No complaints. YMMV