I did some research and testing on this a while back, here's the quick and dirty....
Beginner -> slow steering response on-center, countersteer help
Amateur -> faster steering response, minimal countersteer help
Professional -> even faster steering response on-center, no countersteer help
Simulation -> limits your ability to countersteer at all, quite severely, which makes this mode a complete misnomer and highly unrealistic.
I use Amateur setting with my wheel undercalibrated quite a bit. This seemed to be the best way for me to get a linear, quick response from my wheel without overdoing it on-center (in Pro mode my wheel will shake side to side when I try to go straight).