If you can store more energy then you should do that first. The decision to pull more weight should have nothing to do with anything other than your physical ability to control it accurately. The more potential energy you have, the better off you are. You can always put on a heavier arrow too if you want even more penetrating potential.
My example was to show that if you equalize the KE of each arrow, that the heavier one would penetrate more. If the variable of kinetic energy is eliminated from the equation, and yet the heavier arrow pentrates further, then why?