The other thing I have against a 15" shaft kicker on my boat is the tiller arm hits the corner of the transom when in the down position so having the powerhead 5 inches higher helps that problem a bit. You can get to a point where the tiller arm angle is such that the tiller arm gears can jump out of alignment which is a very bad thing. At least that is the way it is with my my model. Also, while underway I don't have to lean over quite as far to tilt up, adjust speed or work the gears on my old dinosaur ('75 model)....some of us can't afford (nor justify) fancy schmancy linkages and electric start, etc. etc.
I also agree that either a 15" or 20" will work when mounted right as Willy has proven. However if you had the choice, why not go with 20"? Unless you just have a 15" laying around or found a super deal on one? JMO