Think Tireless and I from the threads we had a while back get the best fuel millage with the largest boat with the opti's but never seen anyone with a e tec in our size post for a twin motor boat just remember the thread.
Tireless was hitting about 2.2 to 2.3 if I remember correct. I get 2.4 mpg with the newer motors than he did at cruising speed with the opti's and both are 31 footers.
Someone mentioned high voltage load on cranking. They have that one right. Marina stiffed me on the wiring and left the old EFI wiring on it when the motors were bough with the newer Opti harness. Ended up having to take it to Fountain and have them install the correct wiring cause it would not crank so if you can crank a e tec with a rope, that has my vote right there. I carry a Honda EU 1000 with me any time one of the boats are in the water but not for that reason. Just safety. Once we change and got the right wires on it she has never looked back..
Have not looked at the charts but fuel millage is becoming a really big issue and its only going to get worst so that has to be considered when buying now. .
Love the big boats like Propbender is getting but 650 gal tanks for those are way out of range for middle class, and my worst fuel day burn was only 78 gals to date and top speed is 60mph. Seen others post with opti 300s on the 31 and they hit 70.
What I've read with as few as moving parts as the e tecs have they are one hell of a good built motor and I run a Rude on my 21 Hydra Sports (not a e tec) but it might be one hung off the back when and if the other motor goes. Local dealer for rude is a nice marina.
The 31s Tireless and I have are light and get up really fast but the opti's are a little noisy on the back end thats another plus for the e tecs.
I don't think you can go wrong with either. How long each motor is backed by the factory would also be something I would look into and how far away from your house is the nearest dealer that can work on which ever one you go with and do you trust them. Love my optis but the Mercury dealer I have in EC will never see them again. Everthing has to be considered... Good luck.