Which sleeping bag should I take?

You would probably be OK with the Hydrogen as long as you don't mind sleeping in a lot of your clothes. It can easily get down to 25° at night in mid-September in the Sierra above 10,000' and I have seen it get colder. When I go there that time of the year I take my Marmot Helium and I sleep sufficiently warm without too much in the way of added clothing. I have used a lighter bag with extra clothing clear into October but got a little chilly as the morning approached. Of course, I am in my early 70's and don't sleep as warm as I once did.

" When I go there that time of the year I take my Marmot Helium and I sleep sufficiently warm without too much in the way of added clothing."

+1 with a WM Summerlite.

Oops, I goofed. I confused Charles' Helium with his Hydrogen. I was voting for the 30* option. Sorry.

Thanks guys. I'm starting to think that is the safest bet.

Just to make sure everyone's on the same page… Helium is 15*F, Summerlite is 32*F…

A lot of it is so personal… Kind of depends on how you like to sleep, too. I hate being cold at night and tend to error on the side of a warmer bag. Recently, though, I've used a 30*F bag even for temps around freezing. With a down vest or jacket it's been fine.

Also kind of depends on solo vs traveling with someone; slightly more safety margin with another in the mountains. (Not meant as a solo-travel flame, I've done a lot of it, too.)

I think if you're uneasy about whether or not the 30* bag is warm enough, then you should probably bring the warmer bag. If it's more of a "Hmmm, I wonder…" question, I'd probably roll with the 30* bag. Personally I'd probably take the warmer bag on a trip in the mountains.

My half-cent.