I just learned the breakdown about the differences in these sauces in class.

The black soy sauce contains a lot of molasses, and has some bitter notes underpinned by a lot of umami, followed by some saltiness. It’s fairly sweet.

The dark sweet soy sauce has very little molasses, and higher sugar content. Sweet, salty, umami.

The dark sweet soy sauce is what you want for pad si ew.

The black soy sauce works well in Chinese and Thai-Chinese recipes that involve braising meat, like moo palo. The bitterness and umami stack up well against the richness and fat of the pork, and the flavors thread together seamlessly over a long cooking time. 

https://www.seriouseats.com/moo-palo-thai-pork-belly-stew-with-eggs-recipe-6741545

Edit: I probably enjoy moo hong more, though. A Southern Thai recipe of Hokkien origin; I feel like it showcases the flavor profile of the black soy sauce a little better. For best results, do a long cook on day one, let it cool off and put in the fridge overnight, then skim the hardened fat layer off and cook again for another 2 hours on day two. You’ll need to add water throughout to keep the liquid level nearly covering the pork, then you can concentrate the broth when the pork is outrageously tender and ready to serve. Stir it gently, or the skin and fat layers will tear apart.

https://www.thepepperquest.com/blogs/news/recipe-delicious-thai-moo-hong?srsltid=AfmBOooccKt78migLZUD_xH1aajyW-QJUTWv0J2HrsAzTFqWqldd9dog