I like mostly straight for heavier axes or offset handles for smaller ones. Most people haven't had access to these because they buy store handles, whereas I make my own because I'm poor. Pics
Don't know the science behind it, and don't think much of it anyway. I like these shapes better than the more curvatious handles, curvy handlesfeel like they are trying to swing into the ground/my feet. The offset handles especially feel like they are floating, very accurate and not fatiguing. And for what its worth, everyone who has picked up an offset (such as the little 2 1/4 pounder pictured) has commented on the balance in a positive way. In use it works out even better.