IN 2016 I've been in Hua Hin from 26th of April to 8th of May. Back then I had the only kite I owned with me - an 18m Flysurfer Speed 4. I'm about 93 kg. I could kite every day from about 10:30 am to 2 or 2:30 pm. From then on the wind was too much for that kite and my skills
It started with 10-11 knots in the morning and increased to about 15/16 knots at the given time and on till about 5:30 pm or so . Around noon or 1 pm the other guys launched their kites - mostly in the 10 to 12 m range.
But it is correct - end of march usually goes into the end of season.
In general I loved Hua Hin (been to Thailand before - Bangkok, Chiang Mai, 2x Koh Samui) because of... laid back atmosphere, enough nightlife to have fun without being forced into it, very good food at more than reasonable prices (especially seafood).
Not been to Pranburi but it should be much quieter and less crowded but as Toby said with less social life/nightlife.
I liked it so much that I will be in Hua Hin next year (2018) from 2nd until 21st of march... This time I will take better/more stuff with me. In addition to the 18m I'll take my 10m Speed 4 and two new and better boards - Spleene RIP39 (139x42,5 cm) and SESSION 45 (145x46 cm). So I think I will be well equipped
One downside in the gulf of Thailand is that you can encounter jellyfish that sting. Not the deadly ones but bad enough to ruin your day or maybe more. They (kite school) recommend wearing long sleeve lycra (what I do anyways to protect my skin from the sun) and kind of "leggins". I think if jellies are there I will wear a long running tight. Might look weird an uncool but if it saves you day/vacation... The thing is that no one can give a "forecast" or statistic when thejellies pop up. In 2016 there were none of them (none that I have seen).