[willy]

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