Interesting question

I'm not aware of any hotel within BKK which would qualify as a resort, rather than a city hotel. They all cater mainly to business people. Most of the hustle-bustle of the city is left outside, but you can still feel it.

The one that names itself a resort would be the Mariott Bangkok. I haven't been there, so can't advise on it. But given your choices, I'm not sure it's up to your standards.

Having said that, you can relax very nicely around the pool and in the spa of the luxury city hotels, but all other public areas, including restaurants, are business like, and made for people to move, not to relax.
The rooms in most hotels do not offer a balcony, which isn't really surprising, given the heat, humidity, noise and dirt outside. But don't expect to sleep with your window open in Hua Hin either. (Heat, Bugs).
Also the beaches I've been to in Hua Hin are not that inviting, so the advantage of the sea is not all that great for me either.

Since it's your first trip to Thailand, you may be pleasantly surprised about the levels of service and friendlyness, and especially about the food. Even a city hotel should be fine as your every wish is read from your eyes.
As soon as you leave your hotel, you can still enjoy BKK, but only as long as you have more or less unlimited time, especially to get to your destination.

Without the early morning departure, my scale would be about on "break even" between BKK and Hua Hin, but with the early departure, it means you have to leave Hua Hin in the middle of the night, so you'll be stressed again by the time you're on your way to Myanmar.

Therefore, I'd choose to stay in BKK, or would maybe consider the much closer Sea Side alternatives on the eastern shore of the bay, such as the new Sheraton outside Pattaya or the Royal Wing at the Royal cliff resort. Driving time should be about half of that of to Hua Hin, and while neither is in the league of the Mandarin Oriental, both should be more relaxing than any BKK city hotel.