I think the bottom line here is that you can't compare claimed horsepower ratings by manufacturers! They vary wildly due to the way they are calculated.

Also you need to verify if those weight figures are for engine only or engine and muffler.

To get real data for a comparison, you really need to compare rpm figures using the same fuel, plug, prop, break-in procedure and ambient weather conditions.

Then you will have a meaningful comparison.

If you look at Clarence Lee's reviews of both the OS FS-70 II and the Saito .72 you will see that he used the same plug, fuel and prop and the same break-in procedure. So this would be good data to work off for a comparison.

You are right about the weight advantage going to Saito, but based off the data presented in the above mentioned reviews, you would be wrong about power output.

Now when calculating the power to weight ratio, the lighter weight of the Saito may contribute to it having a better ratio.

But it does not seem to have more 'outright' power than the OS.

Also there are some cases where the little extra weight of the OS may be to your advantage. e.g. When using it in a warbird that may be tail heavy. Better to add the weight to the nose with something that is usable, than to add lead weights.

Also remember that the OS engine uses a sleeve in the cylinder whereas the Saito has no sleeve and the cylinder is machined from the cast block. This may be an advantage to the OS also in that it would allow a new sleeve to be fitted to the engine after the old one is worn to add more 'life' to the engine.

The Saito also has a plastic backplate which some view as inferior to a metal one.

The bottom line though is that whether you choose the Saito or the OS, you will have one fine engine and in the real world you will probably notice very little difference.