I was just looking at weights of the engines - the OS 110 pumped 4 stroke is nearly 8oz lighter than the 120AX with muffler, that is, if you can trust OS's specs on Tower's site. 8oz savings in the nose plus balancing weight in the tail is a pretty serious savings on a 110 size airplane.

The 120AX is an awesome engine. The big difference in performance between that engine and the YS 110 is the throttle response; with the OS you need to give throttle well in advance of a maneuver and spool the engine up for power into a climb. With the YS, add throttle when you need it, almost instantaneous.

I ran a 15x10 on the YS and a 16x8 in the 120AX - the 15x10 just lagged in throttle response way too much with the 120AX (for me anyway).

All of this was in a Venus II. If you are working with a lighter airframe, I imagine the OS 110 4 stroke would have ample power. Furthermore, you get the reliability of an OS engine, which I was told and have learned, is priceless. You can't beat an easy, reliable engine that gives you nothing but stick time.

I ran the 120AX with a Perry Pump. I had problems, most likely due to garbage in the carb and only a season of tuning experience. The carb on the 120AX is not designed for that kind of fuel pressure and it is more sensitive to tuning. I had a dead stick in my best round at a contest and decided that was enough for me, and switched to the YS. Of course I had problems with that, too, and now am going to fly electric. I think with a pumped OS 4 stroke, I would still be flying the Venus and very happy with it, that is, if it had enough power.

Another thing is that you can still run 30% nitro in an OS engine for some extra power. The YS engines are supercharged, so they are more powerful, however some of that extra power theoretically has to be due to the higher nitro content and thus increased fuel consumption. Most people (like the guys not flying pattern) are running 10-15% nitro in OS engines so it's even more difficult to make a comparison.