Open Question: backpack question, i have 18 inch torso.?

so my torso ended up being 18 in. (measured correctly this time) i am a male. 5'11, 150 pounds. and the backpack i am looking at right now is a s/m and a m/l sizes. and it says s/m 14.5-18.5 and m/l 18.5-21. now should i go up in size? everything is exactly the same on the pack, same Liter size and everything, just the torso size is different.

Source:
http://answers.yahoo.com/question/index?qid=20110730230939AA4mksB

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