Tell taxi driver, you want the fare to be via the meter. Tuk Tuk is more of a novelty and a minimum 3x of taxis fare (meter rate). Should you want to avoid feeling been ripped off then it is best to avoid taxis that insist on flat rate ; avoid tuk tuk.
Basically, taxis driver wants a flat fare and TUK TUK cab (3 wheeler) is excessively overpriced. We paid 150 BHT for a short tuk tuk ride from our hotel to a restaurant ; probably less than 20kms. Decided to take a taxis back to hotel, thought we bargained well when we brought the price down from 200 BHT to 100 BHT. Next day, we took a taxis to chatchuka market ; a much longer journey (25 mins ride) ; this time the fare is by the meter and we were pleasantly 'shocked' that the fare came up to 50 BHT only !!! On the way back, the traffic was so bad that it took a much longer time ; probably 50 mins and yet the fare was around 50BHT. We decided to 'compensate' the driver.
It seems that the taxis fare is by the distance. not time based ( I may be wrong here though).