why are you even discussing this?! And how can you ask for "proof" about who's the better pianist?
I really like this site, but sometimes it's just hits a low that I was unaware of... Why is it so freakishly important for you people to have the answer to "who's the best?", and why are you arguing about it?!? No musician with a somewhat clever head can argue that any of those two were amazing musicians. Chopin wasn't a great fan of Beethoven, but he recognized his genius anyway.
And that you argue that one of them is better, because he had bigger repertoire?! And sometimes not even bigger, but just "He played the newer generation of composers, therefore he was better". what is that?! Since when is music about that?
And you can't possibly be serious when you say that he didn't play as well when he was old as when he was young? Oh, Captain Obvious strikes again! I can assure you that most concert pianists will play better when they are 40 then when they are 85. Music is not about "You're just as good as your latest concert". If anyone make one great recording of Waldstein, and 4 bad, you can't judge it on the 4 bad ones. And who would care about the 4 bad ones anyway? If Rubinstein had a memory slip in Chopin when he was whoever cares how old, why the crap should we care about that? HE WAS WHOEVER CARES OLD! Most people who are that old can't remember how to put on pants, and he managed to play the Chopin sonata! And since when is music about wrong notes?!
Don't be so extremely shallow and superficial, so that wrong notes becomes more important than the poetry.