Tuk Tuk is usually more expensive than Taxi as they do not run by meter and hence will always quote you a very high price. Be sure to bargain. If you know the rough price of the area you are going, you can bargain to a price where you are comfortable.
If you have not taken Tuk Tuk before, do try it at least once. Its fun and thrilling, just that you will have to bear the fumes from the exhaust. Not a very clean journey to your destination if its raining!
At times, taking Tuk Tuk is faster as they will go by shortcuts. Especially during peak hours at Siam Square area, it is very hard to get a cab back to Pratunam. Hence I would opt for Tuk Tuk for the fastest alternative, paying him around 80 to 100baht.
Usually older drivers who wear the blue or grey shirt are more honest and will be willing to take you. Many young drivers will just quote you an atrocious price.
Scam Note: Some tuk tuk drivers may tell you that they can bring you to MBK for 20 baht. Do not ever board! Once you board, they will tell you that MBK is closed and will bring you to some jewellery place where they will get commissions. Hence, do not board when its too cheap and when its too expensive. Want to escape all this hassle, take the taxi!