I have one with a OS91FX, very nice. I have about 200 flights on it. The plane weights 6lbs 15ounces. I've never flown it with a piped OS60FX but I suspect it would be a smoother, cleaner flying plane. The plane feels over proped with the 91. With the 60 you could run a smaller prop and also the pipe would get some engine related mass further back.
The piped ST90 might be good , but you'll still have to swing a lot of prop with this engine. Depends on what you want to do, fly pattern or do rocket launches.