I had 4 days in Krakow, and am very glad I did both tours … on separate days.
I LOVED the (obligatory) guided tour of the Salt Mines. I hadn't been to anything like it before. The tour was very nicely done by a very engaging (English-speaking) guide. The salt carvings and figures are simply amazing. Every time I look at my pictures, I can't believe they were done in salt. A few of my favorites were Pope John Paul II, The Last Supper and The Chandelier.
I'm not clear on whether you're considering doing both in the same day, or each as separate day trips. I went to Auschwitz and the Salt Mines on separate days and am very glad I did. It would have made for a long day to do both. Plus, for me, after Auschwitz, I wanted to go off and contemplate, reflect on what I had seen and heard. I wouldn't have been in the right state of mind to fully enjoy the Salt Mines afterwards. And it wouldn't have felt appropriate, for me.