i have no direct experience of this on your size of boat so i can't comment from personal experience. so take the following as generic advice on the subject.
i would max out the HP rating if you think you can lift the weight onto the boat...........50kg plus is a lot of outboard to carry about, especially if you launch far from the car. ofcourse you can make a trolley/wheels etc to make life easier if you wish to do that and can get it in your car.
i would think the 15 yam would be fine at getting up on the plane and very comparable to a 20hp 4 stroke.
once you are on the plane then the 4 stroke benefits would be show more in that they are quiet and fuel efficient. that said, it is only 15-20hp we are talking about here so none of them will drink fuel at an alarming rate and i personally wouldn't give a hoot about that unless you are doing LOTS of hours, like many hundreds per year. it may be a concern to you though.
another factor is are you going to carry an aux engine? if so you may hit the transom limit quickly, you may even max it out with the 20hp 4 stroke? check your makers plate for info on that and get a tape measure you make sure you have room before you spend a penny on anything........the 15hp 2 stroke leaves enough leeway to get a 2-3hp outboard should you wish,weight wise anyway if not space wise.
although not a honwave, this may give you some idea-