I own (and occasionally make) too many different whistles to have a clear idea of which is best (or what "best" means!) ... but I bought a Dixon two-head set ('whistle/fife') in black polymer at this year's (Australian) National Folk Festival.
The whistle (obviously tunable) works well and keeps its tuning firmly in place. With the fife head (bought because I'm always telling myself I should work at being better on fife / crossblown) I found that I could play much better than on other one-piece metal and plastic fifes I own.
I think Tony Dixon is doing a lot of things right ... and, for a hand-made, multi-format instrument - the price was very good.