Bangkok is not a relaxation city. I really like Hua Hin, but if I were you, I'd spend 3 nights there, and the last night in Bangkok because of the logistics of your flight. And for a 7:20 flight, even from Bangkok, you'll be leaving the hotel before 5 AM - so it would seem pointless to spend a lot for a top tier hotel that you won't even use the facilities of. When you're looking at an early moring flight, the most important thing is a comfy bed and convenient logistics to the airport.

If I were you, I'd arrange for a late afternoon departure from Hua Hin the day before your flight,then head for the Novotel Suvarnabhumi Airport hotel, and stay there the last night, where you could get a good night's sleep, roll out of bed around 5-5:30 and get your flight.

As for Pattaya as an alternative - it does have one advantage - it's only a 80-90 minute drive to Suvarnabhumi and a drive that, at 5 AM, is much less susceptible to traffic than the drive from Hua Hin or Bangkok city to Suvarnabhumi. But Pattaya is an entirely different sort of resort - VERY much a playground for single adult males - which either you want, or you don't. If you're NOT looking for that, I would avoid Pattaya like the plague. Of course, you may exactly want that - in which case it would be the best choice from a logistic standpoint - although the resort hotels in Pattaya are at least 1-2 cuts below the nicest ones in Hua Hin and Bangkok.