I believe you are correct about the FFB Sensitivity, 1 is most detail, 10 is least, but it’s been a while since I’ve played with that setting, so I’m not 100% sure I remember correctly.
It's pretty telling for the quality of FFB on GTS that people can't agree if 1 or 10 is the highest sensitivity setting.
This is a quote from the manual about sensitivity:
The greater this value, the more responsive to changes the steering wheel will become. When this value is lower, steering becomes gentler when driving in a straight line. Increasing or decreasing these values allows you to adjust the controls to suit your driving style.
To me, that implies higher is going to be more sensitive to detail and my testing agrees.
What should i choose for best experience?
Whatever you like. It's your game and your best experience might not be ours.