You did not state how big is your sailing boat and where it is moored. I can only assume that it is less than 30' on a swing mooring.
Fuel consumption should not be the primary issue since you are not expected to use your outboard a lot of times in a sailing boat.
Power should not be the key issue either because a 9 Hp instead of 15Hp will only lower your top speed from say 6 to 4.5 Knots.
Noise and smell are issues to be considered but I shall not rank them too high.
I believe the most important issues are weight and size. I already assumed that you are in a swing mooring and even if you are not, storing the motor inside the cabin is always better for maintenance and secuirty. In that regard, weight must be the key factor. It must be light enough to be handled by one single person. A 2 stroke will have the advantage.
If storge is tight, like many sail boat, a 2 stroke is generally smaller although not by a great extent.
Depending on where you are, the local agent to support your motor is also a factor for consideration since you are buying it new.
In my personal experience, a 15 hp 2 stroke is too heavy for one person to transfer it to its bracket. However a Johnson 9.9HP weighs the same as the 15 HP version may not solve the problem of weight. I suggest you should shop around using the above criteria and see what is available in the market.