I would definitely say the Gnomenreigen. On the grand scale of Liszt's entire pianistic output, ranging from fairly difficult to unplayable, Liebestraum is at the very bottom, while Gnomenreigen is about the middle of the road, being an etude after all. Compared to other pieces, the Liebestraum is moderately difficult, about a 7 out of 15 (15 being Liszt's Feux Follets or Ravel's Gaspard de la Nuit or whatever equivalently insane piece you want to throw up there). The Gnomenreigen is about a 10.