I am 5'9" and roughly 150-160 lbs depending on the season and I am trying to make the same decision between the small and medium. I'm planning on taking this jacket to nepal and pakistan for some high mountaineering and I'm wondering if this application changes which jacket I should go with. Should I be looking at the medium more because of the capacity for layering? It just seems like it has too much room in the chest or like it may be too long. I'd also be looking to use this jacket for technical routes, so there is a big premium on freedom of movement without extra fabric to snag, hinder, etc. Help would be super appreciated!

I am 5'9 160lbs.....I would go with a medium....

How low should I be looking for this jacket to come down in the front? For the arms on the size small, the sleeves come down the last knuckle on my thumb when I am standing upright with my arms at my sides, and the front hem of the jacket is about 3-4 inches below my hip bones. I figure these generally may be designed to be worn with bibs, so maybe this height is reasonable? My major concern is for rock or ice when I'm reaching high and at full extension, is it reasonable to feel any tension in the jacket across the back of my shoulders? Feels like these are kind of nuances and details that I have never had fully explained or addressed to me...

I think to get the right length in the arms and in the torso that you are going to want a size medium - particularly in the front the jacket is cut on the shorter side so I'd worry that the small would be too short. I will say that you are going to have some extra room in the chest but it will give adequate room for layers, and I don't think that it would be baggy. If it's an option for you, you might be best off ordering both sizes and return the one that is less optimal. If the arm and torso of the small fit you, then I'm guessing at your size/weight that the small would be the better option. Sight unseen though, my vote is for medium. For reference, I'm 5'9" (maybe an extra 1/2" on a good day) and 170 pounds, and I wear a medium.