Which piece is easier to learn(Moonlight sonata or Grand Valse Briliante from Chopin)?

If you are talking about learning them in their original key/score they are both very difficult. I found Moonlight Sonata slightly easier ,mainly because you are ment to play it very slowly, and that makes it easier. The best way to learn any piece is to play it very very slowly over and over until you get all the notes and rythme right,then gradually increase the pace until it is at the pace intended by the composer.This is how most professionals do it. Even the most famous concert pianists learn every thing very very slowly at first, they memorise all the notes, then gradually increase the pace until it is perfect.Most important enjoy it.
Good Luck