both are very well made engines...as far as the power to displacement goes..i would have to lean towards the ys...but saito's are well liked for there light weight...at least the 72 is, it weighs in about 1/2 pound lighter than the O.S 70....i haven't owned a YS in about 10 years, but i do know for a fact that saito's are operator friendly as far as critical needle adjustments go....i would think that the new saito 100 and the ys 91 are very equal.....as far as power...and haven't checked but would also think the saito would way somewhat less......