I think Passenger Departure tax (700 THB approx £14) is only payable if you leave the airport - not for Transit passengers,
You may see a slight decrease in Fuel surcharges (YQ) if you book a transit flight as opposed to a stopover.
Personally, I think a brief stopover in Bangkok provides a great contrast to Singapore and Hong Kong.
You can confirm specific taxes using the ITA website (or mobile apps for iPhone/iPad, Android).
Hope this helps.
Regards, Graham.