if you don't care, then saying either one of these instead of saying "i don't care" is stupid. if you don't care, then you wouldn't be inventing wordy ways of saying so.
"i couldn't care less" can't mean anything other than "i don't care" so it is always a stupid thing to say.
it's like when someone does something trashy and someone criticizes them by sarcastically saying that was "real classy". is paying attention to a trashy person's trashy behavior and using sarcasm to insult them a classy thing to do? no. so people who do this sarcastic "real classy" thing are themselves behaving without class.
so if you don't care about something, don't feel the need to say so. if it's expedient to declare it, then do so succinctly. using a wordy communication style to express your lack of care is inconsistent
anyway, the fact is that most of the time, someone does care a little. "i don't care" is usually hyperbole. so this whole argument people like to make about "if you could care less, then that means you care a little" -- well, yes. that is almost always the case. this isn't an interesting argument. people making this argument would be better off pestering people who say "i don't care" after they've already said or done something that demonstrates they care at least the tiniest bit
another fact of the situation is that idioms don't follow logic. there are plenty of illogical idioms. so this whole routine of people criticizing anyone saying "i could care less" by explaining to them how it's illogical is just faux-intellectual BS. there are other illogical idioms that you could use that wouldn't trigger them. and the notion of arguing against all illogical idioms is ridiculous as well. so what are they doing? it's pointless and stupid.
so the only thing left to say is that it's possible to use "i could care less" to express something other than "i don't care". you can use it to threaten to care less, for example. but it's a conversation-stopper because of how many idiots it'll trigger into saying "you mean you COULDNT care less" so it's not worth using
both phrases should be avoided