Inaction for me. I'm trying to "say yes" to things more often these days. I also disagree with the no regrets philosophy. Don't think it's even possible, really. I can't imagine not regretting a single thing.
I have had both situations happen and I probably regret some actions taken more so. Both situations were not pleasant at the time but with hindsight they probably all played out as they had to in the mess we call life.
I regret things I did do more often. There isnt much I regret not doing because I just do most of the this I want to. Edit: spelling