If you had to choose between getting permanent flight or lifesense for a swashbuckler (via Imp. Eldritch heritage and the Phoenix and Possessed bloodlines, respectively), what would you choose, and why?
Depends on your party.
If you have someone who can cast Fly on you when needed then Lifesense.
Otherwise I'd take Fly. Fly is always going to be useful, while Lifesense is only useful when there are invisible/hiding enemies. Even then you don't need it to locate them, it just makes it easier (and even then it doesn't work on all enemies).