I went from a 12 year old but, great shape, 18 ft ARC with a 90 hp Mariner to a 6.5 Ribcraft the a 150 Honda. I considered the same thing, upgrading the engine, to a 115 hp newer technology outboard. When we trialed the 6.5, the differences were BIG. The ride, interior room, sea handling abilities that the three extra feet in length provided were significant. Just an engine upgrade would not have changed any of those things. The fuel consumption actually decreased +- 4 miles per gal. to 5 or more mpg.
I also can still launch by myself if need be.
I guess you have to decide what's important to you. I was still able to get good resale on the old boat, but for how much longer I don't know if I had kept it for another 4-5 years.
Good luck with your decision. If you do upgrade, do check out Ribcraft.