I've previously looked over No. 7 for a few days and found it quite easy, but I was wondering if No. 8 is easier.
This is only my opinion about List Transcendental Etudes.Personally I put #8 as my last option to learn. TE5 is easy to memorize and requires less stamina than #8. However, that's as easy as it gets with #5, the real demand with #5 comes from the interpretation, not so much the technique. TE8 on the other hand is the exact opposite of TE5. I'm not up to the interpretation part with TE8, so I can't say anything about it. Otherwise the answer is - go with the one you like most and learn that. There's no right order to learn the etudes.
Wilde Jagd, when played at the correct tempo, is harder than what everybody says IMO. Feux Follet is supposed to be one of the hardest, having played both, I found #8 way harder. The amount of stamina/endurance needed is just insane, just when your wrists start to burn the hardest is still to come. Never played Heroica but it seems much much easier, except maybe the octaves part. If you have strong wrists and have no trouble with octaves, I'd say go with #7. If you're looking for a huge challenge go for #8 or even #4 maybe ?