Try em all....and judge for yourself. I recently went over to the ''Dark Side''...prop in the water boats. My poor airboats ( I have two Airforce...the other one has an OS .55AX with the same size prop) justs sits now....but I will fire them up one day...I am so consumed by my new boats....a Deep V mono and a Hydroplane. I am going to give my brother one of the airboats to have someone to run with and I am excited about your boat. You could say I have the best of both worlds...maybe....but your progress will speed up my renewed interest in the airboat hobby as the other boats engines ae very finicky at times....and I have had more bad days than good....compared to my airboats.
I will end up running what I got and buying 2 or 3 more I'm sure. I like to pick the brains of the guys that have ran these engines to get an idea of what they ran in a plane. In a nutshell, its the same game. Its all about thrust getting the beast moving. Smaller props spin faster but at a sacrifice in thrust (in some cases) because of the smaller diameter. Obviously if the engine can spin the smaller prop better than the bigger one, it may net more thrust..
I don't consider water-propped boats the "dark side". Its still a boat, and I would like to build one of these with an outboard at some point... I'm building the airboats because I have a bunch of airplane engines that were free or cheap. With some ingenuity, skill, and time you can build a air-propped boat to go pretty darn fast.. Although a high-end .21 inboard will probably spank even a light fast .61 airboat, (to me anyway) its the fun of building something from scratch, have it float, and better yet run well. I think my boat collection will end up being 3. 1 flat bottom, 1 hydro, and 1 deep V outboard. If the hydro turns out good, I might build a bigger one for a bigger engine... I'm gonna make a few mods to the foam and start drawing soon. I will post pics of the build as I go.