Well, that's a tough question to answer. How often do you get to see your friends in Paris? Do you travel there or do they see you much? If you don't get to see them, than I'm sure a night in Paris after three days in Belgrade would be good. You would see both.

However, if I were in your situation I would probably go to Belgrade unless I wasn't planning to go to Paris anytime in the near future and they were my dearest friends I've always wanted to see Belgrade. And you are right, you might not go back there.

Good luck with your choice. Either way, it's a great choice to have!