Well, I know it seems as though I'm being unduly captious not to accept as fact that the Chopin piece is more difficult, but I think that standards of proof as to what constitutes the "difficulty" of a piece have not been developed with sufficient logical rigor. There need to be more objective standards of what constitutes "difficulty". How do we measure "difficulty"? To say that the Chopin piece is more difficult just because examination boards say it is strikes me as being rather arbitrary. I doubt very much if every judge who sits on such a board has exactly the same opinions about the relative difficulties of various pieces as every other one does. Not everyone even agrees that the Chopin piece should go faster. I've seen the Moonlight Sonata at 88 half-notes per minute, versus 80 per minute for the Chopin.
Actually according to my editions:
Moonlight (Augener's Edition) is marked Presto Agitato and 1/4 note = 168
Op 10#4 (Alfred Masterworks Edition) is marked at Presto 1/2 note = 88 (which if counted in 4 would be 176)
Therefore the Chopin etude does in fact need to be played faster. Add to that the LH of Op 10#4 is much more difficult than the LH of Op 27 #2 as Beethoven tends to have single notes in the LH, Chopin on the other hand has the melody quite frequently, it also has chords, octaves, and pedal too.
Then examine that Op 10#4 is an exercise in passing the melody between hands which although moonlight does as well, the melody is single notes, played much slower. Also note Op 10#4 has a key change as well.
Then the musicality aspects: Musically moonlight is not difficult. That is why it sits at grade 8. It does not need an experienced pianist to interpret the work well. Chopin on the other hand does require a very skilled pianist to do musical justice to the Etudes and this is why it sits at LTCL level which is roughly equivalent to 3rd year of an undergraduate degree in piano performance.
2nd to 3rd year undergraduate level compared to last couple of years of high-school level of grade 8, one does not need to be a genius to see why Op 10#4 is regarded (and apart from you, universally accepted) as being the more difficult of the two pieces.