Hi Jeanne,
You may get more responses to your question if you start a new post on the subject, but since we're here now...
Plitvice Lakes in Croatia is a very scenic area, the main activity being hiking and enjoying the outdoor scenery (not much else to do, although it is beautiful). Mostar is a town in Bosnia that was hit hard during former Yugoslavia's war in the 1990s, and activities there include visiting local historical sites, shopping, eating, etc. Shell/shrapnel damage is still quite visible on the buildings and streets.
If I had to choose (and I've been to both) I would pick Mostar. But that is because of 2 things: 1) my husband and I aren't big hikers (the Plitvice hikes are easy, though), and 2) we got engaged in Mostar and therefore it is very sentimental.
But, as neither of my 2 reasons above are particularly relevant to you, the decision will have to be based on which one sounds more interesting and is more suitable to your travel style. Or, which one logistically fits into your overall itinerary better.
If you can do both, though, that to me would be the best option!
Feel free to PM me with questions, I'm happy to help if I can!