This one is easy, 61-85 size really means forget about everything exept a .91 engine. Todays .91 engines are bored out 61`s and since they are of the exact same physical size and share the same mounting space and bolt pattern, then why settle for the least powerful one????

For 3-D flying you need real power and a .91 will be much better that a .61, or .75 (never heard of a .85)

OS 91 FX is great, Magnum/ASP XLS 91 is just as great only less expensive, Super Tiger 90 is said to be great too