I would proritize op. 25/1. It's much more of a "classic" of the piano literature than Un sospiro. Not having it in your repertoire is like pretending to be a specialist in English literature and not knowing Byron's poems. Also sospiro is not "nicer" than 25/1 in my opinion. 25/1 is one of the most amazing pieces of romantic piano literature.
In terms of difficulty I also disagree with what has been said. Ok, getting the notes right with op. 25/1 is easier than with the Liszt, BUT let any pianist play the first few bars of 25/1, and the experienced critic can immediatly tell whether the pianist is bad, mediocre, good, or superior. It's sort of easier to hide technical and especially musical deficiencies with Un sospiro imo.
This said, the best advice came from Berhard, as so often.
So I would go with op 25/1 first and second with Un sospiro (or Waldesrauschen, which I think is a more interesting etude).