I have the basic cheap Soodlum's mello D, a Susato set that has a mouthpiece and three tubes (Bb, C, D), and a Shaw Low D. They all have really distinct characteristics, that I use depending on the song I am playing. For example, one of the songs I perform is called "The Walker of the Snow" and I got it off a Davey Spillane album. Spillane plays an eerie, moody interlude on the pipe. I use the Susato whistle, which has a sharp chiff, and play and finger it as you would a pipe. That is, I don't tongue the notes as much as I run them one into the other as a piper would do. I also use half-fingerings to get the wail of a pipe. Brendan Nolan has a ballad he wrote about Grosse Ile and the Irish emigrants in the coffin ships. On it, I use the Shaw low D.

The Shaw low D can be a miserable cur to play. If you are new to whistling, stay away from it until you get your fingering and breath control down. The holes are a long way apart and it takes a lot of air to play. On the Shaw, getting to the very highest notes is tough to do cleanly without a lot of practice.

I also would strongly recommend the webpage link that Dan gave towards the top of this thread. If you subscribe, Dale Wisely will also send you a weekly or so email which has invaluable reviews on the various new products. The homepage has a good primer on the various whistles.

Mick