If we are talking only about what happens when the arrow strikes an animal, then momentum is king. Arrows kill by penetrating a broadhead. no penetration, no kill. period. Kinetic Energy is much more at home with today's well constructed rifle bullets. It was in days past, with inferior cup/core lead bullets, where large bores and heavy bullets (high momentum) killed more animals. High KE bullets produces larger shock waves to be transmitted through the chest cavity, a very different internal phenomenon than an arrow/broadhead.
Also think about the construction of the Arrow. carbon fiber is fantastic at absorbing energy. Think carbon fiber bicycles, they are both light and dampen the shock from the road. The internal damping (energy absorption) of carbon fiber is higher than just about any material. Although carbon fiber arrows are hard to break, are they really what you want at the moment the arrows strikes the animal you've been stalking for a week? I know lots of archery hunters who are switching back to hunting with aluminum arrows because they have had bad experiences with light/fast carbon arrows not penetrating.
However, there is more to it than just penetration. Flat trajectory from higher velocity has a lot of merit. And as mentioned in a previous post, the correctly matched spine for your bow is going to be the best all around combination. As with everything the best place to be is a good happy medium with all factors.
