I prefer februar and march because no rain (usually) and not so packed like december (christmas).
In the highseason there are of course more farangs in phuket, but on the other hand much more girls, too. (Many of them have a break in low season. Stay at home in isaan etc.)
Last year in november/december i had a nice room for 500 bath a day. So you can find a cheap room in feb, too.
I myself don`t like the low season because to cloudy and sometimes raining.
But each to its own.
Hugo