First of all, which Chopin concerto are you talking about? To me, Chopin's 1st concerto (in E Minor) seems more difficult than the 2nd (in F Minor), but that's just me. Overall, the Rachmaninoff concerto is, in my opinion, superior to both of Chopin's concerti and is more difficult. I still love the Chopin concerti, but they are rather immature compared to Rachmaninoff's, especially the lattter's 2nd concerto. With the Rachmaninoff, there's a lot more coordination with the orchestra that's involved since the orchestra plays a much larger role than with Chopin. There are also much larger chords and more difficult passages. The Chopin concerti are still immensely difficult, but Rachmaninoff's seems to require much more out of the pianist. So, to clarify, you should learn the Chopin first and then progress to the more difficult Rachmaninoff. Whatever your choice is, good luck!