Interesting. I play(ed) both (and the 1st ballade as well). Technically I find the Fantaisie rather hard (certain sections are just difficult to settle), and the Polonaise-Fantaisie easy (and I'd wager, of all of Chopin's major works, this is technically the easiest). But then again, I played the Fantaisie more than 1 year ago, and my technique has changed a lot in that span of time.