Hi pianistimo,
Both the E flat and E natural sound fine and each has their adherants. Chopin was so changeable! It might be that while composing he heard the E natural in his "mind's ear" and notated it as such. After all, on the second beat of that very measure he actually made that preceding E a natural--so why would he necessarily change it back to a flat within the same measure? Later on, maybe he penciled the flat into the student's score during a lesson in the moment, believing that he had really meant E flat to be consistent with C minor all along. We'll never know. It would not suprise me in the least if in his public performances he played it both ways depending on his mood!
To answer your question on whose side I'm on, my response will very much surprise you. Here goes. When a composer writes a piece, once it's finished, quite often he or she is off to another project. If the composer is also a pianist and conductor like Rachmaninoff, Bernstein, Previn, etc. this can happen frequently, especially if the composer doesn't select the work to perform publically in recital. So it gradually recedes into the composer's background owing to the composer's busy schedule.
The pianist is different. When a pianist studies and plays that composition for a very long time, he or she gains insights into the piece that the composer may or may not have realized. In time, the pianist becomes a curator of the piece--a solemn responsibiliy. I'll boldly add that the pianist sometimes comes to know the piece better than the composer did in creating it. That is, the pianist discovers potential that was not apparent to the composer originally. I once heard a recording of Donnanyi, a wonderful composer and pianist extraordinare, playing one of his own pieces--it was obvious to me that he didn't realize the powerful potential of the piece. (He was one of my teacher's teacher's teachers, so I don't want to be too hard on him, haha!)
Two of my favorite composers are Rachmaninoff and Ravel. I've posted many of their works here. I have Rachmaninoff's recording of his piano transcription of his song "Lilacs", for example. (My posting of my own rendition is on page 10 here.) While I enjoy Rachnmaninoff's rendition, it's a tad... well, businesslike. I infuse it with a more intense romanticism. If I were to play it for him, if he were still with us, I believe he'd approve that interpretation, although it certainly wouldn't influence his own. We know that he allowed Horowitz to resynthesize the components of his Sonata No. 2 between the 1913 and the 1931 editions, and even change the ending of the latter, for instance. And when Perlemuter was studying all of Ravel's piano works with the composer, Ravel imparted suggestions, but did not try to control Perlemuter's interpretations. A final example: I was in a competition in Boston in my youth. (I wish I were still in my youth.) The jury was New England Conservatory of Music piano faculty. One of the pieces I played was Bach's Prelude and Fugue, No. 2 in Cm in Book I of the WTC. Despite the score, at the end of the Fugue, I played the final beat adding my own Picardie Third as ornamentation. Not one eyebrow flew up. I surmise they must have taken for granted that Bach was all about improvisation, and that if what I did would have been acceptable in 1670, then it should be acceptable in our day too. They advanced me to the finals.
So, I believe this: The pianist has a sacred trust in reproducing the composer's intentions by being loyal to the concept and details of the score. But if the pianist has insights that justify a slightly different and justifiable approach--while not being idiosyncratic, subjective, in bad taste or totally contrary to prevailing performance practices--then the pianist should unhesitatingly present his/her interpretation accordingly.
Topic: chopin prelude #20