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

I have. Answer is simple.

I don't really see them too different from each other. Wishing bạn hadn't đã đưa ý kiến hoặc should have đã đưa ý kiến means bạn want to change your past actions. In both scenarios bạn 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 đã đưa ý kiến something.

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