Waist. Most of the bib contact area runs at the waist. If you think about it, torso won't matter so much in the position you assume on the bike. I like my straps a bit snug standing b/c they are generally just right when on the bike.

Google the bibs too before you pull the trigger. Some makes (PI) run a bit large. Others, mostly the "euro" cut run smaller. I'm typically a 31 in jeans (I know jean sizing is way off from true waist) and I wear the medium aka 32-34 waist in volar bibs (those are euro cut as far as I know).