Are you sure the Schumann variations are just as hard as Mendelssohn variations Op.54? How would the schumann variations compare with other brahms variations?
To determine the difficulty of a piece, I would look (if I could) at the amount of time it would take for an average accomplished pianist to learn the piece to concert performance level assuming the pianist had never seen the piece before. If piece A takes 200 hours to learn and piece B takes 400 hours, then I consider piece B to be twice as hard as piece A. Of course pieces that have a longer performance time (excluding any repeats) will take longer to learn, everything else being equal.
Q: Are you sure the Schumann variations are just as hard as Mendelssohn variations Op.54?
A: No. I would have to spend more time looking at this. Even then I would not be sure. Without spending more time, I feel fairly confident the Mendelsohn Op 54 and Brahms Op 9 are in the same ballpark. If I had to pick, I would say the Mendelsohn is a little tougher than the Brahms.
Q: How would the schumann variations compare with other brahms variations?
A: The Op 24 Handel variations and fugue and the 2 books of Paganini variations op35 are of course in a totally different ball park.
The Op. 21 No 1 is a gem!! Without repeats it takes about 10 minutes to perform (16 minutes with repeats). I would say this is perhaps in some ways significantly more difficult than the op. 9 (played without using the "piu facile" alternatives). Overall perhaps about tied with the op 9.
The Op. 21 No 2: Like the op 9 (and the op 54 of Mendelsohn), there are no repeats. This set takes about 7 minutes to perform. I would say this is the easiest (although not easy) of the Brahms variations.
What are your thoughts?