I think the idea of friction on the arrow shaft is somewhat overblown. Yes, I think it does play a role but, since most of the resistance is against the broadhead blades and since living flesh naturally pulls away from a cut, it's a minor role. I'd say there is far more resistance on plastic vanes going through a wound channel than there is on the shaft itself. Another good reason to use feathers instead.

So, I'm going to say weight is more important than shaft diameter. Now, if you take a small diameter arrow and add weight to it, then it would likely penetrate slightly better than a fatter arrow of equal weight. If that smaller diameter arrow is also a good quality carbon shaft, then you've got the best of all worlds.