Personally, I'd prefer to see both types of torpedoes - flat damage (photon) and decreasing damage (plasma).
The trade-off becomes range vs damage potential. Energy consumption can be based on potential damage at some intervals, meaning longer range or higher potential damage both increase power requirement.
Speed of both should be a design factor as well. Trade speed vs range vs damage vs accuracy. Add in a factor for damage fall off rate (maybe based on square of max damage?) to allow for differentiation - have this affect power consumption.
Speed and range effect launcher size, damage effects power required and maybe size to a lesser extent.
