First of all, 2 things; 1: a happy New Year! and 2: both these pieces are within the same scope of difficulty, but for different reasons. The Polonaise contains quite a lot of repetitive parts especially rhythmically, so getting the hang of it musically is more easy. However, it contains excruciatingly difficult 2-hand leaps across the keyboard with very awkward passages right after landing your hands after such leaps, with some rapid chromatic scales in between (however, these are relatively easy although they sound daunting).Having said that, I am 100% sure the Ballade will be much more beneficial to learn. It has on offer these very same difficulties, but offers a myriad of others too, while also offering loads of time to relax, because significant portions of it in between, are lyrical and not technically demanding at all.But, it contains all sorts of keyboard acrobatics apart from large jumps, like arpeggios that are written out in twisted ways (like going a few steps down, hitting a 2-note interval, moving up again, another 2 note interval, moving down again and repeat on a different octave for example), and extreme tempos (the coda is a tough nut to crack even for seasoned pros).Apart from that, I think the Polonaise is physically heavier to play (making you tired faster) while the Ballade is more mentally demanding to play (as it is musical storytelling of the highest order).Overall I think both pieces are in the same difficulty league but, if you mean to tackle a piece that is more difficult than anyone you did before, and you want to do this in order to become better as a pianist, the Ballade will be more useful I reckon. In both cases I would recommend Alfred Cortot's editions of these works, he offers priceless musical insight and also means to combat any technical difficulties that arise while learning them. They're available in English translations through www.broekmans.com (international sheet music and CD web shop) if you need to purchase them. Good luck, hope you conquer them both ofc