Long shaft 20" is better. As for the 9.9 and 15hp debate the 9.9 makes as much torque if not more than the 15hp in the OMC flavor. So at displacement speeds that a kicker is gonna push your boat the difference would be nil to nada between the two. Prop would be more important, sailboat or push prop would be the way to go. I have a Yamaha high thrust 8hp kicker that has pushed boats to 23' with ease in wind and current, it has the right prop.

B

Short shaft was best on my V, if you are putting it on a bracket that raises and lowers, you just measure before you install obviously to ensure where the prop is. Never had mine come out and cavitate even in the ocean.
On a fixed bracket then you might need a long shaft as they are generally mounted higher so the engine can tilt all the way up and clear the back of the transom.
Either will work fine, just depends how and where you mount it.
As far as HP, what I have found with my sailboat friends is this, the 8 & 9 HP motors will work as the previous poster says as you are at hull speeds only, but they have found that if the current is strong or the wind is kicking up the 15 with the right prop is better. Fuel difference is very minor.
If you are using a 4stroke engine definetly go with the 15

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. :you:

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

It worked fine. Sort of hard to reach down and the tiller was not fully straightened out but I wanted to use the 25 for other purposes too. Later I had a 9.9 4 stroke that worked much better with a long shaft, but that was a single purpose kicker. Suppose it depends on if you have other uses for the kicker that require a short shaft.