The 35RA is great in any application. It's compact and runs strong and reliable. The 30 will be fine in a Stick type sport plane and even better if your on a budget. If you don't plan on flying warbirds or narrow scale cowls then the 30 will be fine for most sport planes.

Get the 35 and a NGK CM6 spark plug with a Xoar wood 18x10 and you won't regret it!