I don't know if this helps but since I was assistant in optical
mineralogy for 5 years, I guess I did aquire some practical knowledge
about the behaviour of microscopes in the hand of students.
My preference goes to the Leica Laborlux 11 or 12 Pol microscopes
because even though they are rather expensive to acquire, they have
proven to be good value on the long run. Indeed, unlike other
microscopes (no names here), they are rather choc proof and do not
need to be readjusted very often.
What however makes the great difference with other microscopes is
that the Leica microscopes can be taken apart and repaired very
easily, which saves a lot of trouble. (I had to send a microscope
only once back to Leica for repair)
Of course the Leica optics are also about the best you can get.
Good luck and all the best,