I have both. The OS91 makes more raw power but is a little more finicky to set up and run. The YS 80 is a bolt in and go, good power and the same YS reliability we got from the 61s. Use a gear ratio around 7.8-8.2 for the 91 and 8.2-8.5 for the YS. They will work on other ratios but after testing I think these are the best, though others think differently.