I have the 91 Saito and the os 70 surpass. I actually swapped the 0s 70 in my p-47 with the Saito 91 and noticed no concernable power difference between the two other than the saito seems to handle the bigger props a bit better dut to its displacement. Having 3 Saito's and 2 os's I would recommend either. If weight is a primary issue go for the Saito. If ease of use is the primary issue go with the os. They seem a bit less sensitive to proper adjustment. Both will last forever.