The Bb whistle is listed on C&F as a “standard” key whistle, but an A whistle seems to be fairly common. Which is more useful?
In my limited experience, Bb’s are nice mostly for Scottish music because they allow you to play along with highland pipes. A’s are more common for Irish, if I’m not mistaken. It’s easy to play in the key of A on a D whistle, though.
TW
If you’re playing in a D session, the Bb whistle is useless. The A whistle is really handy for a couple of tunes (“Pull Out the Knife and Stick Her Again” in E minor comes to mind) and fun to use on many more. It’s also a great way to practice playing in a different key signature while still playing with other people. (ie you can get key of D from what would be the key of G on the D whistle.)
If you were in an Eb session, the reverse would be true.
On the other hand, for playing with pipes in Bb, for playing with singers, or for solo whistling, the Bb whistle rocks. It’s just exactly the right key for whistling – not too high, not too low.
A.
Bob Pegritz
I would say a Bb is more usefull simply because its one of Generations best sounding whistles and is easily available and fulfills any Eb playing requirements. D whistle sounds best for D tunes.A whistles are a little too strident in the 2nd octave for my taste to play D tunes.A Bb Gen with a little bit of fine tuning is equal to any of the expensive designer whistles and have received nothing but praise on this forum.Peace,Mike 
TR
I agree with Colomon, it depends on the situation. Many of the older(and several newer) Irish recordings use Bb pipes and Bb concertinas so the Bb whistle would work with them. However do what I did, get a Bb/A Sindt combo…they play and sound terrific and you get 2 wonderful whistles for an incredibly decent price…then you are prepared for anything!!
Thorton,
Which whistle you choose may be influenced by what tunes you want to play. I noticed your from Atlanta, at the base of the Appalachians. A whole lot of Applachian Mountain music, Blue Grass, and Old Time fiddle music is in the Key of A. My favorite A tune is Ashokan Farwell.
If your playing a lot with folk singers, the Bb might be good too. A lot of guitarist will play a tune using Key of G chords but playing with 3 capo’s to get the key closer to a singer’s optimal range. This results in the song being affectively in the key of Bb.
So which do you pick, why live in denial of your WhOA … Pick both.