The closer one is to the soundhole (when felting down on the strings with the chordbars), the better the tone, no matter what wood is in the box. So the 15s just sound better.

But the 21s-- because they permit accompanying in more keys without a harp switch, are better for accompanying (or in my case leading) music in a wider range of singable keys.

But if the main course is going to be bluegrass (or oldtime or oldtimey), and the range of wanted keys is therefore narrower, wal, ole pal, the 15 oughtta do ya proud.

OTOH the 21s sound purdy good with an onboard pickup, which has a tone adjuster in addition to whatever tone-adjusting you want to do in the amp.

That's the real deal on the difference there, Wes.

~Susan