Who should write my LOR?

I am having trouble choosing between 2 dentist to write my LOR. I have heard that some schools want to know how many hours you have shadowed the dentist that writes your lor? Dentist A: Has been a LONG time close family friend, and a very well known established dentist that attended the university I want to go to (UMKC) but I only was able to shadow him for 2 hours. Dentist B: Dont know nearly as well and pretty fresh out of dental school, but I have shadowed him for 40+ hours.
Source: Student Doctor Network - Category: Universities & Medical Training Authors: Source Type: forums