Re: i/o or outboard
I've owned both, but most of my boats have been outboards. I/O boats do usually seem to do better on fuel, there's no oil mixing and are more like car engines if that's what your more familiar with. If it ever came to having to work on the motor while on the water, the I/O is the better choice. Not all I/O set ups are cramped to work on. My 1971 trihull has a lift lid set up in which the entire rear section lifts and exposes the motor out in the open, you can sit right along side of the motor and do just about anything that needs to be done with the lid open. It can be near impossible to work on some outboards from in the boat, two of my boats are set up in such a way that you can't even reach the outboard from inside the boat without climbing up on the rear deck and hanging over the stern.
I/O set ups are heavier and do take up interior space, there are more moving parts and there are boots or bellows to maintain to prevent water intrusion.
There is no hull opening below the water line to worry about with an outboard boat.
Being that most outboards are still two strokes, they also tend to make better power vs. weight. I have two nearly identical boats, both are early 70's 17' trihulls, one with a Merc 120 I/O, the other with a 120 O/B. The I/O boat does about 35 mph on the water and the O/B boat has enough power that I probably would never open it up to find out. The I/O boat weighs in at about 900bs more than the O/B version. I would venture to guess that the my Mercruiser 120 set up weighs nearly double what my Evinrude 120 V4 does. The outboard boat is also better balanced and not as stern heavy.
My 120 O/B will use about 4 time the fuel as my I/O boat, but since its lighter and quite a bit faster, the O/B boat does get run harder, so it's hard to say exactly what the fuel consumption difference really is. The I/O boat does better at slower speeds, it can run at an idle smoothly where as an older 2 stroke runs better only at higher RPMs.
I tend to prefer an O/B but if I were buying brand new, I would consider either. I am more weary of used I/O set ups simply due the fact that I am near saltwater here and it can be pretty costly to try and repair or reverse years of saltwater damage. An outboard can be swapped out easier.
An outboard also doesn't have as many moving parts to fail. An outboard's final drive is fixed, it has no universal joints, bellows, or pivot points to maintain.