I've been studying Chopin's "waterfall" etude for about half a year now, but still miss many notes (around 1 in 10!!) when playing them at the actual speed. As for Un Sospiro, I've been playing for a month, and can get through about half of the etude without mistakes.  If I'm lucky, of course. 

So, basically what I would like to know is which one is better for an intermediate amateur?  I'd like to pick just one for studying purposes, as I haven't got enough time for both. ><

Any suggestions would be helpful, thanks!

On an unrelated note, what do you guys think about Haydn's Piano Sonata, H. XVI?