Taxis are generally cheaper in Sri Lanka than to book their own car hire. Also, the traffic is very chaotic and therefore it is much more relaxed to drive easily.
Compared to train, bus and tuk tuk, a taxi is a much more expensive means of transport. For this the taxis are generally equipped with air conditioning and the vehicles are generally in a very good condition.
If you want to book a taxi directly from the airport (or to the airport), you can do it very well over the following page: