We have been to both places and would say two full days in Memphis and maybe a third if you want to just unwind. Stayed at Tom Sawyer across the river and loved it. Ten minutes and you are at the visitor center and river park. As for Nashville there is more to do if you are a country music fan, so we spent more time there. A day at The Hermitage is and some of the surrounding area is nice along with the whole Nashville scene. Check out things to do online in each place and then you will have a better idea of how much time you need in each place. Hope you have a great trip.