I've played the Polonaise, but only "dabbled" with the Scherzo. I think they both are quite difficult, but great pieces! Saying which is "harder" probably depends somewhat on your own physical strengths/limitations, but I bet they are pretty comparable. If you learn the Polonaise take extra care to stay relaxed during the left hand octave passage in the middle. It's the one thing that may prevent you from playing it - tensing up will only keep you from playing it up to tempo!