I haven't flown the mini, but I know that the 90 size flies great. Depending on the distance between the engine and firewall, you may have to modify the front end to get the pitts muffler to fit. The tuned pipe will provide more power and (probably) less modification. Both will provide a "clean-ish" setup.

Pitts muffler is cheaper, but produces less power, and you may have to hack-up the nose...

Stuffing a gas engine in my Showtime took a little work, but I think it was worth it.