A very pointful thread I reckon. I think the C#D has a lot going for it for the standard sort of fiddle/box repertoire. It seems to preserve a lot of the diatonic in-out bounce of the old one row, very suitable particularly for the Cork/Kerry sliabh luachra(sp?) type of thing. Seamus Begley plays one, and also Jackie Daley I think; plus that really famous siabh luachra musician, Johnny somebody whose name momentarily eludes me, (I'm sure someone will oblige with it in a minute).
B/C is good for decorations and speed, but it loses some of the bouncy dancability that I love.It has certainly moved from being a rarity to being the most popular one at the moment.
   D/G boxes have been used for Irish music, but they don't seem to have really caught on. They are quite a recent innovation in the Britiash Isles, and have proved eminently sauitable for English tunes(not to mention Tex Mex).
    Of course, a really accomplished player can get more or less the sound he or she wants from any instrument, but for lesser mortals it's good to choose the box that most suits the style you are aiming for.