We are here now and undecided which one to do. If you had to chose one which one would it be and why?
Mostar, since it is more different to Dubrovnik than Kotor.
Read what is said aboth both Mostar and Montenegro in this Bloomberg article, than decude:
Save Montenegro/ Kotor for a holiday in itself.
1 day for Mostar is plenty, to fully appreciate Montenegro you will need more than 1 day