I know hardly a word of either.
However, I think I can safely say that Sindarin is the best to learn. It has by far more material available, and eventually becomes the language of all the Elves of Middle-Earth. It is originally, of course, the language of the Sindar, but the Noldor learn it because of their knack for languages. After the decree of Greymantle () it becomes the absolute Elvish vernacular.
Quenya is the Elf "Latin." It is the academic lanugage of lore, and is only used in poetry or very formal situations, as far as I know.