is it better to use baking chocolate or regular high quality bar for this recipe?

This recipe is literally just chocolate and water. Use a chocolate that you love.

I would select a high-quality chocolate bar that I like. After all, the recipe is chocolate + water (with the optional addition of a flavoring agent). The flavor derives entirely from the quality of the chocolate.

Per the headnote, the recipe's success depends on the fat ratio and each chocolate has different amounts of fat. I would likely take notes on the actual chocolate (including cacao percentage) and how much water I added/extra chocolate I added.

Best of luck.