We have twin 40's. If I were to repower I would go with twin 50's. I can just barely plane with one engine when heavily loaded. Twins IMHO allow me a greater safety margin, currents here, in my cruising area, often run well over 4kts. A kicker that gives me 4-5kts might not be enough for safety in bad weather and opposing currents. I can spin the boat in its own length, use opposing propulsion, one in forward one in reverse, to maneuver in tight spots, etc. I don't know the exact costs but two Honda 40's are about $10,200 not twice as costly as a single 90 which is about $9300. When you add in a 9.9hp kicker at about $2300 you actually spend more for the single and kicker. Maintenance cost differences are not as great as you might think. I have a total of six cylinders, a 90hp has a total of 6 cylinders, I think. Yes I have to buy two fuel filters, oil filters, impellers instead of just one but the bigger ones needed for a single cost more. And, don't forget the kicker has maintenance costs too. Yes there is some weight penalty but I'd check the actual figures. Modern engines are so reliable I'd be comfortable with either a single or twins but I am overall more comfortable with twins, I like redundancy in mechanical systems that can mean my safety. Example, I am going out Deception Pass against a strong 7kt flood and an engine dies. I push throttle forward on the other engine and proceed. With a single plus kicker, I leave the helm, go to the cockpit, put the kicker down, start it and....