Hi: this is really a guess without hearing you play these. However the answer rests on how good a performer in front of people. Here is my take. If you can play without error then go with the waltz. Its a great piece , but its very delicate and exposed. There is no where to hide with a wrong note . It is very transparent. On the other hand the Polonaise is a little more forgiving as far as rhythm and notes go. So you know you best of all. Good luck with your choice.
Kelly