‎"Have you ever wondered which hurts the most: saying something and wishing you had not, or saying nothing, and wishing you had?"

I have. Answer is simple.

I don't really see them too different from each other. Wishing you hadn't said or should have said means you want to change your past actions. In both scenarios you feel regret so they're pretty much the same.

I would say saying nothing hurts more, because I'll always be beating myself up for being a coward, and I'll always wonder what would have happened if I had said something.

If I did say something, I will have learnt from my mistakes and won't make the same mistake again.