I've not flown the Decathlon, but have aerobatted a Robin, and wasn't keen on it for learning aeros. The reason is you're learning to coax a not particularly capable aeroplane around the sky, so are effectively spending a good deal of your time learning the aeroplane rather than the figure. I went from the Robin to a Bulldog and due to the extra capability I spent my time learning the aerobatic figures and not having to learn the plane, which more or less went where it was pointed.