Troi is a psychologist and perfect for the counselor job.
Guinan on the other hand is rather an advisor, for anyone who pops into her bar and especially for Picard. Picard does not like counseling, when he has problems either Troi has to follow him (GEN) or a friend has to open his eyes (Lily in FC). And when he has non-personal problems, tough decisions to face as captain, he will consult Guinan.
Let's take e.g. Worf having problems with Alexander. Troi will give him explicit advice while Guinan might serve him a prune juice and indirectly remind him that he is not a typical Klingon, thus showing him that he is also half-human in a way.
That's kinda the difference of Guinan's and Troi's style.