I don't really see them too different from each other. Wishing 당신 hadn't said 또는 should have said means 당신 want to change your past actions. In both scenarios 당신 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.
