For the simple reason of the closed cooling on the 496 Mag and HO motors, I would aim that direction. Over the long haul, you will be miles ahead and have a favorable power package for resale in a saltwater market as well.
As far as maintenance, I have twin 496HOs, in a 37' Active Thunder with full 6 year warranties. The 496s are supposed to go 100 hours before you even change the plugs. I took mine to 150 before I did mine. I am now way over that hour mark, with nothing more than annual sea-water pump impellor changes. They are a bulletproof package especially for easy cruising like I do.
As far as fuel; try to contact Dave M here on this site. (I will send him a PM to look at this thread.) He has a 24 Superboat that he went from a 454 to a 496HO. He should have some good direct comparison info for you on fuel. (He has a big propeller on his beanie.)