I concur. Get a D whistle, which will allow you to play easily in both D and G. You should know that the best key to play a whistle in is the one with three fingers down (i.e. G on a D whistle). This is because

(1) Most tunes go below the key note (usually no more than 3 notes)

(2) The top of the range on nearly all whistles is a bit squeaky, so it's hard to play the full 2-octave range evenly

Cheers!
Ian