chashint

I have not found the Magnum 91 or 70 to be underpowered compared to the same size OS engines. Between my brother and I we have several of each, we use 14x6 on the 91s and 13x6 on the 70s and with the same fuel they are all about the same. Less than 500 rpm from the strongest to the weakest and the line-up is mixed so it not like the Magnums are all bunched at the bottom.
The Magnums run every bit as reliably as the OS and we are very satisified with them.
Now after having sang the praises of the Magnums, I will also echo the sentiment that if the price difference is only 50 or 60 dollars I would buy the OS or the Saito. While I have no personal experience with the Saito engines I would say that the OS engines are just a little more refined than the Magnum engines. All of my Magnum engines have large RPM changes with one click of the HS needle, this has not really presented any issues but in three clicks you go from to rich to right on to to lean .... the engines hold the needle setting very well and you don't have to fiddle with it, but the OS needle lets you dial in the RPM you want and it holds the needle setting too.
So that is why I say the OS is more refined.
If there is not a sale on the Magnums and I was choosing between OS and Saito that choice would be greatly influenced by the plane the engine was going in. If it is known to need nose weight then the engine would be an OS Surpass, if the plane is known to need tail weight then it would be the Saito.
I don't have any large displacement engines but the 70 and 91 size OS and Magnum engines have very little vibration, I cannot comment on the Saito engines since i do not personally own one.