I would say that it depends on when in December you are looking at , the nearer to Christmas you get , the more chance of some flights being fully booked. Thai is the most expensive airline, but also has most flights and is pretty reliable , "low costs" airlines are becoming very popular but this also means availability may be less.
If you have 3 nights in Bangkok you will at least have a bit of flexibility about which flight you take . Personally , because I fly in from the U.K , I usually book with Thai ( got stuck in Bangkok once for almost 24 hours trying to get on a flight on the Queen's Birthday
) , so now would rather have "confirmed" flight when I arrive.