I had a SS prop on my merc 15 and bent two prop shafts with it. Bottom line according to my repairman is the merc 15 has a wimpy prop shaft and prop strikes cause it to bend more easily than other motors. Stick with the aluminum in this case.
Have always run them on Evinrude 15's ,better performance .The prop has more cup and i was able to set the motor up a little higher. A must if you are running in sandy bottom conditions where aluminum won't last a season. I believe they only had one size for the 15.