I have never had an extra so I can't really compare. After long hours of flying the sim Knife and my real Knife I can tell you that the real Knife hovers 20xs easier than the sim Knife. I could never make the sim hover right (even with tons of rearward CG). The sim Knife stays pretty locked on in a knife edge. My real Knife does a great Knife edge but the sim is just a tad better. The trick to that is getting your ailerons in the right place.
The sim does perfectly match the way the Knife handles at low speed and how it lands. When you bring the sim in to land and think "no real plane will land this slow" think again. The real Knife is great with a nose up attitude. The real Knife can land on the rear wheel and keep going down the field that way. So the real Knife is better than the sim in all most ways.
Anybody out there that can compare ?