I like Alkan's op 39 very much, tbh.
As for the difficulty of Le Preux, my votes in the poll I cited were: it, the Concerto for solo piano, Fumagalli's lh only Robert le Diable, the 1837 TEs as a set, and Hexameron. I've either learnt or looked at, reasonably seriously, at least half of the items on the options, so I'd like to think that I answered with my eyes open, so to speak. Successfully playing them live is a tremendous feat imo.
Wow. How much time did it take you to do this? Did you get them to performance level?
I'm noodling around with the 1837 Feux Follets now. It seems certainly friendlier to the hands than the Paganini, albeit more "musically" complex. I wouldn't say either of them is much harder than the other I think.
There's kind of a reason I took an interest in the Paganini 4. The early versions of the later (revised) Transcendental etudes and Paganini etudes have more notes in them, but most of them sound the same to me (admittedly I haven't seriously listened to them all but this seems so).
So if they sound the same yet are harder, what difference would it make to give yourself a hard time? The exception is that 4th Paganini etude.
In the original set, Liszt had two versions of it, one with double notes and another with much less notes. I believe the third revised one in the collection is the easiest (while still not being easy of course itself), and, boy oh boy, did Liszt no longer include the 1838 double note one in his revised Paganini set. Lol.
So the Paganini 4 seems unique in this way, as it's the only one I know that differs greatly from its revised counterpart.
That being said, how do you see the 1837/8 etudes from their revised versions in terms of difference? Are there sections in the early ones that markedly differ from the late ones? Are there particular etudes (or parts of them) that you prefer in the earlier sets?