Left hand for the pen here.
I immediately went for left and tapping on my right once I got a tablet. Reason for change is because as a left hand, you already have a better control and movement coordination of your writing hand. Whereas if you use right hand, you have to start from the beginning as if you learning to write on a paper with your right hand as a first time awkwardly. Personal opinion only though.
Tapping with your right hand will feels as if you just started playing osu as a newbie long time ago. But as you already have played osu, your streaming and tapping abilities will return pretty fast in a few days.
In the end, whatever your comfort is, go for it!
Good luck training fellow left hander!