Okay lets say I have an 18' Triton.. Let's say I have 2 motors to choose from. Let's say a 150 Mercury EFI and a 150 Mercury ProXS OptiMax... How much more powerful is the OptiMax??

This is completely an uneducated guess but I would think the real difference would not be in "power" per se as they are the same hp.  I would expect the big difference to be in the efficiency of each motor.  I'm sure others on here have way more experience as to the difference of each motor.  My guess is that they are going to have slightly different requirements for maintenance as well as overall effeciency (effeciency=mpg!!!).  Because of that, you might get a few mph out of one motor over the other, but probably at a higher purchase cost.

Good luck, and I'm sure the experts will chime in soon.

There really wouldn't be much difference in power.  The only differences is engine wieght which means you may need a different prop than the what the other one may require and mpgs!  Saying this the Optimax usually gets around 5 mpgs while the EFI gets around 3.  With gas prices these days the optimax would be the way to go.

Aside from which might be faster or more powerful, the Opti is way more efficient and environmentally friendly.