It really depends on your strengths... typically Ravel's stuff requires a VERY subtle touch to convey the writing... typically Liszt requires a lot of Bravura... but the touch required in La Campanella kind of reminds me of that in the Ravel.
I'd probably say that because La Campanella is an etude, you'd probably run into more hurdles throughout (mostly related to jumps), and the workout on the outer fingers for the right hand is probably going to take a while... but in the Ravel, there are some articulations and pretty ridiculous cadenzas that require more than just 'bashing the notes'.
You'd probably want to look at the Ravel before the Liszt. However, just because you see threads saying "Jeux d'Eau" is one of Ravel's easiest pieces (debatable), it is by no means EASY. If you are comparing it to the ENTIRE sets of Tombeau, Miroirs, Gaspard, then perhaps 'Jeux d'Eau' can be considered 'easIER', but not "EASY' as in a beginner/intermediate pianist should look at it...
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Look at both; in the Liszt, try to find the 3rd entry of the B major theme... that will give you a good idea of the technique you will have to bring to learn this piece. Also, check out the variation with the 4-5 trills (I tried messing with 3-5-4-5, but my RH can stretch quite a bit).
For the Ravel, I find it more of an exercise in reading... the harmonies are more obscure, and it's a little harder to retain some of the passagework IMO. So, it's really difficult in a different way (for me). Playing both at a VERY SLOW tempo, I'd probably have a harder time sight reading the Ravel.
But ya... both are AWESOME pieces. Try not grading pieces in the same way people would grade levels in a video game... the repertoire at the level of both these pieces is VERY expansive. Some have easier moments, some are brutal. You'll find instances of both of those cases in these two pieces.