It is a fact (not opinion) that a heavier arrow shot from the same bow will result in both more kinetic energy and more momentum and therefore all things being equal will result in greater penetration. If you want to attribute the increased penetration to the increase in KEthat's fine. If you want to attribute it to the increase in momentumthat's fine too. KE and momentum are merely the result of calculations based onthe measureable quantities of arrow velocity and mass. From a given bow, the facts are quite simple. If you put on a heavier arrow, the veolocity goes down and both the KE and momentum increase thereby resulting in greater penetration.The rest is opinion. Along with reduced velocity as a result of the heavier arrow comes increased trajectory. You have to weigh the gain you achieve in penetration against the negative effect of increased trajectory. Where you decide to draw that line is very much a matter of opinion. Personally I like to lean to the heavy side when picking an arrow.