Sack them.

Cooldowns prevent you from drawing a high level fight out forever. So the purpose of buying X more turns is to multiply by Y damage per turn to get (X*Y) more damage.
Also, my gut feeling is that the effective proportional gains aren't the same. That is, X% increase to HP doesn't necessarily mean you actually last X% longer, whereas X% increase to damage output is closer to killing an enemy in 1/(1+X) time.