This version of the Fantasie-Impromptu is less well-known than the more popular version in C minor, Op. 66. It is also shorter, and is in a different key. The C-sharp minor version was written in 1834, while the C minor version was written in 1835.
The two versions of the Fantasie-Impromptu are very similar in structure. Both pieces begin with a slow, lyrical introduction. This is followed by a faster, more virtuosic section. The pieces end with a return to the slow, lyrical introduction.
However, there are also some key differences between the two versions. The C-sharp minor version is more chromatic and harmonically adventurous. It also has a more improvisational feel. The C minor version is more straightforward and lyrical. It is also more technically demanding.
Ultimately, the best version of the Fantasie-Impromptu is a matter of personal preference. Some pianists prefer the C-sharp minor version for its chromaticism and improvisational feel. Others prefer the C minor version for its lyricism and technical demands.