It’s a tough one.
I know Klingon and not Elvish, like Brian Collins. I think I disagree with him: Tolkien gives slightly more opportunity.
- Elvish is a more complicated set of languages than the agglutinative Klingon.
- Elvish is much less well documented by Tolkien than Klingon is. That’s why people are very reluctant to use Elvish conversationally at all (and they put an asterisk next to the grammars done for the films). But that means you have to exercise a lot more linguistic and philological skill to get anything out of Elvish.
- Paramount only used Klingon for the Trek movies, and when they did, they asked the language inventor to do the job. TV Trek have either ignored Klingon, or used the “idiots with dictionaries” approach to language learning. (wIjjup for “my friend”. Where –wIj is a SUFFIX.) So even less chance of remuneration than for Elvish.
I think Elvish would be endlessly frustrating to work on, which is why I was never tempted. (That, and Tolkien’s mythology didn’t do anything for me.) But for that reason, it would be more of a mental challenge.