To respond to my own question: Hat Dalam for sure if you are into a mellower vibe. Ton Sai is super congested with boats, and it's where all the ferries dock so there is TONS of traffic and noise. Also there aren't a lot of places to chill out on Ton Sai side, mostly just one big resort. On Dalam, there are lots of little beach bars with beach chairs, a lot less boats. Both sides have gorgeous scenery and water color but Ton Sai is just way too crowded with boat traffic and the noise/smells it produces. Water on either side isn't super clean, sadly, but clean enough that I felt ok to swim, and the sand is very soft. On Dalam you can find small hotels/bungalows relatively near the beach, though very few are right on the beach.