Well, the other day I sat down and did some work on 8/12. The left hand is tricky, but once you have it really memorized, the piece becomes quite playable, and actually alot of fun.
However, to answer your question, I would definitely back up the concensus-op. 42 #5 is much harder than op. 8 #12. On the scale of etude literature, these are fairly high up my version of the difficulty ladder-comparable to Rach's more difficult etudes tableaux.