My choice is the heavier arrow every time to help with momentum. Penetration is dictated by arrow momentum and broadhead design going inversus thestopping force applied by the flesh/bone. Stopping force comes mainly in the form of friction, and friction goes up exponentially with speed (i.e., there is 4x more friction when the speed is doubled across a given surface). My arrows are just shy of 525 grains and fly at 255 fps from my Mathews Switchback at 30" and 69#. I personally make the heavy arrow choice because I hunt mainly elk, and I never know what unforseen problems are lurking that will shave off my penetration ability. The bottom line is shot placement, but when the unseen twig moves my shot placement into the shoulder, then I want to have as much weight as possible to salvage whatever penetration I can.