The question should be -which concerto is more aesthetically pleasing to me as a performer and which concerto will my talents best serve the composer's intentions. As far as which one is "harder" my humble opinion would be Tchaicovsky. Rach 2 was composed by a pianist who knew all the ins and outs of writing for the piano. Tchai's is a beautiful showcase for pianist and orchestra but not composed with the comfort of the pianist in mind. As someone else said it is more transparent and sometimes awkward.