I've done both. Could/should write an essay on this question.

Most important: I think that the specific qualities of a particular startup or particular lab outweigh the importance of the category "startup" or "lab". Good labs, bad labs; good startups, bad startups.

You can often leave a bad startup faster than you can leave a bad lab.

Anecdote: I'm in a lab now and I'm really glad I'm doing what I'm doing. I like it a lot. It has the feel of a startup, but I'm doing research.

Much more but must go to lab! :)