Settle for massage in Airport. I think anything else, i.e. considering a trip to more central Bangkok, would put you dangeriously close to missing your connecting flight.
Plus there are subbtleties (i.e. bar find fees, suckered into some set-up for poor service, etc) about a Thai SP that should not be rushed.
Enjoy a 6-hour relaxing time in the airport. Come back to Thailand later when you have morre time.