I've just started playing the pennywhistle, and I have a D Oak and a C Clarke. The Oak appears to be chrome over something and sounds great. The Clarke seems to be a tapered, folded over piece of tin, painted bright greeen, and doesn't, in my opinion, sound so good--it has more of a tendency to have odd vibrations in the notes. I have one album of pennywhistle music, Billy Novick's Pennywhistles from Heaven, and he's phenomenal.
I tend to go with the chromatic set idea, since tuning my guitars and recorders is more tuning already than I enjoy. One of the reasons I'm learning pennywhistle is I don't have to come up the fingering for Bb or C# or any other odd keynotes on the fly--I just have to start out with the right whistle.
Also, the pw is addictive. I can carry it anywhere, and play it endlessly, by ear. I've made myself a nuisance everywhere. I saw some kind of new age ensemble on TV awhile ago, and they had a guy playing a pennywhistle the size of my alto recorder--it sounded beautiful. A friend of mine said it was probably a "low" pw, very expensive, and, evidently, "haute musique." I want one!
I've only been playing since February, so all this enthusiasm might wear off, but regardless, I've just learned how to clean and lube my pw, and which ones to play in the studio and which ones to take outdoors, and that's always good to know!!!