As jayhad said, any reason other than weight? Maintenance wise and ops wise 4 strokes dominate. A 15hp 4stroke however won't be a throw in the back of the truck type of motor, it will be permanently seated on boat. Bass pro for sure can't be touched on Mercs. Riverside in St. Albert can't be touched on Yamahas. Both good motors, just some features on one/not the other. Keep an eye on kijiji if you are not in a rush, lots of guys will be selling in the fall. When you go look at them ask them to set it up for you to "test drive", either fill a garbage can with water and put the prop in it, or hook up one of those adapters that runs off the garden hose. Run it through the paces including forward and reverse, check everything. I got a hell of a deal on a barely used Merc a couple years ago in November off a guy in Calgary. Food for thought...