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.
You can also turn this argument around.
Every aircraft, from the C152 Aerobat to the Edge 540, will suffer from various aerodynamic traits like adverse yaw, to a certain extent. Once you start flying any aircraft to the limits of its capability, you will have to work with or against these aerodynamic traits to coax the airplane through a particular figure.
The earlier in your aerobatics career you learn how to use these aerodynamic traits/limitations to maximum effect, the less you have to unlearn bad habits at higher levels.
I'm sure it's great to aileron-roll an Edge 540 on a perfect horizontal line at 540 degrees per second, just by slamming the stick to the side. But it requires far more skill to aileron-roll a Robin on that same horizontal line at 200 degrees per second, where you initially use the adverse yaw to keep the nose up, and later need to counter the adverse yaw with significant rudder.
If you want to learn aerobatics so you can impress your passengers or the good folks on the ground, get the most capable aircraft you can. But if you want to learn aerobatics because you want to learn to fly better, you will be challenged most by an aircraft that more or less matches your capabilities.