Know I getting a Tohatsu, my questions is 15hp or 20hp. According to Manu specs, both are 351cc, both weigh 114lbs, both have a bore stroke of 61x60(?), only diffence is 15 is 5-6000 rpms and the 20 is 5400-6100 rpms. Motor is going on a 14 alum V rate at 640 pounds. With myself and son we...