I've since found a couple of comparison websites that report network coverage and it does sound as though AIS is the better option. Having said that, the other 2 seem to rate not that far behind.
I got a really good deal (A$2.70 - after A$5 promo discount ) on TruemoveH from Klook with 100 baht call inclusions, cheaper IDD code and unlimited data for 16 days - so I'll start with that and see how it goes.