So it may be best to go for "new" girls? Are you talking years when you talk about the length of time in it that hardens them?
Difficult to make a set rule as there are so many variables. What I've found is that 'New' girls are sometimes sweet and innocent and do their best to provide a good service, especially East European ones. However some treat the job as a chore and 'switch off' and are just mechanical.
Experienced or 'Old' girls can be hardened but they can also be more skilful and understand what us punters like.
I think girls and agencies should provide a 'Try before you buy' service. Or maybe a test drive for ten minutes before making a booking. Any girls out there willing to give this a go?