I read somewhere, I think, maybe I was told, that the original cadenza is the big grandiose one, but Rachmaninoff thought it was too much for the 1st mvt so he wrote the quicker scherzo which suits a faster tempo (horowitz, argerich). It is by no means easier, at least the build up to it is much more difficult I think. And if you ask me the 3rd movement "fat chords" marked cadenza(top pg. 77) are the real cadenza. despite being only one measure, those chords are frickin enourmous. Horowitzs great pianists recording of that one bar is incredible.your friendRob