twin or single...
For a difference of only 4 gallons an hour at cruise, I will keep the piece of mind of twins.
That fuel difference, plus the cost of annual gear oil, impellors, filters, plugs...I wouldn't exactly call 'substantial'. But I do not commute with the boat either. I can fish near shore all day long and put only 1 hour on the motors. Everyones situation is different though.
What I would call a substantial difference between the two is being 40 miles offshore and one motor not starting or driveshaft snapping (yes it happens) and it being the only motor on the boat or not.
I can appreciate seeking to save money but if you travel offshore, nothing beats twins. Even the Inlets these days, with 1 motor and it shutting down, you can easily lose a boat to the jetty in less than 2 minutes without power. Been there, not fun. No motor is bulletproof. A motor is just a motor and they can all fail at any time and do. You do see all those boats tied to towboats all day long, right? Those are all single powered boats. That is what the towboats are for and making a good living at it.
Twins get you home every time.