I've flown one on both a super tigre 90 and an OS 91 SurpassII. There is no contest between these two engines, the OS wins hands down. If you use a Super Tigre make sure that you do not mount it upside down. I've personally used three different ST-90's upside down and all they do is flood out, and constantly drip fuel while sitting. I can get them to fly, but not as reliable as I would like. The OS is great mounted any way you choose. I too am a fan of the YS 110 or 120. If you can practice throttle management, you can never be over powered. And the YS throttle response is better than any mentioned so far in this thread.

Mike