Speed, definitely. Energy has a role, as they're related due to simple physics and equations, but speed impresses critters much more than energy IME, all else being equal.
As far as lung shot DRT vs. runners with identical shot placement, identical bullets, impact velocity, etc., I read a study that basically concluded that if a bullet hits at the top of the heartbeat, arteries get WAY overloaded and rupture, causing instant death, where if the bullet strikes between heartbeats, this does not happen and they'll normally do their 20-50 yard death run. I have no idea if it is true or not, but found it interesting.
