Okay I'm gonna put my 2 cents worth in here. Forget about anything that's 4 stroke! Too heavy, too slow, power to weight ratio sux! Go with a pair of 115 Evinrude E-Tech's. Most power to weight ratio if you insist upon going with twins. If would like to be able keep up with any crotch rocket motorcycle, or be at red line RPM at the same instant your throttles hit wide open. Then go with twin 150 E-Techs. They are fuel sippers, and once you hammer them throttles a few times you WILL leave a mess in the drivers seat!!! You may wanna stuff three hundred pounds or so of sand bags under the bow if you go with the 150's.
Lots of good advice here and thanks to you all I have decided to go with twin 150XS Optimaxes at 431 lbs each. However Merc just came out with a 4 stroke 150 that weighs 455. So for a miniscule 48 more pounds I can get 4 strokes. And the cost is about the same as the Opti's. Merc has not posted the fuel curves or noise data on they're website yet but this 4 stroke 150 is going to be a game-changer.