Everything that feels is sentient. Something/someone that is aware of itself/themselves (capable of intelligence) is sapient. Ergo, a cat is sentient. That's not interchangeable, but ST shows continue to use both terms as if they meant the same thing. It's okay for the audience not to know, but a supposed super-brain and (rather pedantic) scientist such as Spock should.
EDIT: honestly, I'm not too sure how self-aware Borg drones are. That's a good question.
i can't recall when if ever anyone on Star Trek used the term sapient to refer to intelligent life. The series appears to use the term sentient exclusively for this purpose or at least overwhelmingly.