"Subtitles assume the viewer can hear but cannot understand the language or accent, or the speech is not entirely clear, so they transcribe only dialogue and some on-screen text.
Captions aim to describe to the deaf and hard of hearing all significant audio content—spoken dialogue and non-speech information such as the identity of speakers and, occasionally, their manner of speaking—along with any significant music or sound effects using words or symbols."
Closed captions also provide hints as to non-dialog audio; song lyrics, musical tones, important noises in the background etc.
Personally I always use subtitles, and closed captions if it's available; there are quite a lot of audio clues that I wouldn't otherwise know are even interesting, that get highlighted - mostly extra detail rather than plot important, but it does key you up to things happening or about to happen in the background.
