At the beginning of the sportsman pattern season I was faced with a 4 or 2 stroke decision and ended up getting the 91FX. The OS91FX with a tune pipe was about the same price than an YS110.
The 91FX with tune pipe weighs 28 oz (795gm) and the YS110 without muffler weighs 27.4 oz (776gm). So there’s little difference. I run the 91FX at 10500 with a 14X8 APC and 15% Cool Power. From what a hear you can run the YS110 at about 9500 with a 15X8 prop, so power is very much the same. So you get a bit more toque but you need to run the YS on 30% nitro, you can not hand start it and even with a starter it can be troublesome, then it needs regular maintenance. You also need a very strong airframe to withstand the four-stroke vibration. The two-stroke is a fuel, fly and forget engine. Just what you need then you want to concentrate on your flying.