Which SS montbell

I would (and did) pick the 30 degree bag. It seems like a lot of people like 20 degree bags for 3 season in the mountains, for those occasional cold nights.

For myself, as a warm sleeper, I've so far found 30 is the right compromise. Not so warm that I'm steaming when its over 40 (as happened last summer in a 20 degree bag), but not so light that it couldn't be boosted into the mid-20s or a bit lower with proper insulation (Montbell UL Down Parka that I always carry).

35 degrees is probably a bit too light to be boosted that low, unless you bring more insulation, at which point just buy the heavier, warmer bag.

My two cents.

I started with the 30 degree #3 ss, test slept in it at 30, with regular base layer, wasn't warm enough. Traded back in for the #2 ss, again slept at about rated temp of 25 for that bag, same sleep set up, was toasty warm. Significant differences in those two bags, draft collar, way more down. Sleeping is so individual temp wise, you may be fine with 30. I can quilt mine no problem in warm weather, up to about 70 for low, then its' too warm.

So maybe look at the lowest temp you would expect to decide. I think they are fairly rated, but just wanted to point out the difference in going lower if your are interested in that direction. Love the bag for a flip flopper side sleeper that I am. 6' 170# long side fits perfectly.