Very good points. Maintenance may be cheaper, but not as cheap as you think. As engines get bigger and have more moving parts, their parts also increase in price. Yes, your initial maintenance costs will be cheaper, but when things break it may not be so true. From what I have seen from Yamaha reports, the 150's and 350 are giving off very similar fuel consumption readings, not saying for every boat this is true. As for the hull, you are correct, that big engine displacing all that force on one location may be a bit for that boat unless it is beefed up. Not sure if that boat will need it or not, I have never really checked out an islander transom up close before. It also depends one how the boat is going to be used. If the boat is used mostly inshore then either is a good option, if the boat is every going to be taken more then 10 miles offshore, twins are the way to go without question. Going off the coast in a single is pushing it. Watched a guy with a single outboard having engine problems 90 miles offshore...had to be towed in by his tow boat company....sure hope he had the unlimited coverage!!! More then likely, if he had twins he could have limped home. As for the fly by wire controls, I think they are the way to go. Not sure how well they will be at first but they will be very handy. Heard of them being rough on the mercs right now, they take a little getting used to. Shifting takes a little getting used to but I am sure that will be something that has the kinks worked out of it. I have not heard much about it on the Yamaha's, hopefully it is a better product off the bat.
Tashmoo thanks for the comment on the custom hull!!