Good points Nate but our original poster _is_ probably totally confused now!
I don't understand where the notion comes from that the B/C allows better emulation of fiddle ornaments. After all they are basically the same instrument, just in a different key.
You can do a five-note two-row roll just as easily on a C#/D, and in fact it's very tricky to do one on an F# on a B/C. So, I don't get it this fiddle ornaments business (quite apart from the fact that one of these standard rolls sounds nothing like the way most people play rolls on a fiddle). Also, the practice of imitating fiddler's trebles (triplets on one-note) was introduced by C#/D players, esp. Tony MacMahon (all right, D/D#) and Jackie Daly.
Another thing: most of the prominent C#D players today (Jackie D and Seamus Begley notwithstanding) are not from Sliabh Luachra and don't play lots of polkas and slides.
The extra bellows work on a C#/D is often cited as a minus by B/C proponents. (Of course C#/D players regard it as giving more character to the music.) The corollary is that the B/C involves a lot more mobility with the right hand. An example: 1st-octave scale of D can be played using 4 buttons on a C#/D. On a B/C, you need 7.
And as Nate hinted at, the greater number of buttons involved makes passages involving high g, a and b more demanding.
So Margaret - did you get it?
Steve