I would definitely recommend leaving the airport.

The only questions should concern the details...like where can you leave your carry on luggage while you are outside (the lounge? a locker?)? What do you want to do (maybe temples in the morning, catch a movie at Siam, maybe a 2-hour massage, some shopping)? How can you avoid paying the outrageous 150-baht fee at the airport ATMs? How early should you get back to the airport, given that you'll already have a boarding pass?