Sofia University is good, also there are some good uni not in the capital, but life and chances for good career is Sofia
About Mostar it is an amazing city
About Mostar: that is not a fairytale. That is 2 towns in one (Croat's and Bosniak's part) and you don't have almost any chance for a good life. Only if you are some extraordinary student or they need a foreign person, which is I think very possible.
I have been there and people are really crazy and divided. They (young guys) are fighting each other on day basis all time long. It's like that almost anybody switching a red line (all genders, ages etc; the biggiest steet in the town), divied a city on two completely opposite sides. It's not town for a good living.
Old town of Mostar (Bosniak's part) is extra beauty and it's nice to see but not to be.