The first time I made the trip from Ekkamai, I made the mistake of getting on a local bus. As I was walking toward Ekkamai, I saw a nearly-empty bus leaving the station with the "conductor" yelling that the bus was going to Pattaya. (It was written on the side of the bus as well.) I had no idea what I was doing, so I got on.

Well, it was not a very pleasant trip - it took forever. And about 2/3 of the way through, that bus just stopped, and they put pax on a different (packed full) bus. I wound up standing, crushed near the driver (None of the "no passengers forward of the line" nonsense in Thailand! ). The one advantage was that the bus actually went to Jomtien Beach, which was my destination.

I believe you can tell the difference because (1) the "good" bus (government bus with a ticket purchased inside the station) has assigned seating and I have never seen one leave that is not full, and (2) the "bad" (local) bus has an orange stripe on the side, while the "good" buses have blue stripes.

If you get the ticket inside the station, you get an assigned seat and the ticket will show the number of the bus, so it is really pretty straightforward. The bus has aircon, and the comfort is fine for a few hours. I usually take the bus unless I am feeling particularly flush with cash, in which case I will take a taxi.