Ditto all that Steve said.
- Is it easier to play fast jigs and reels on a B/C than on a C#/D?
Like superfast? Over 120 bpm? Yes. But for normal sane session/ceili tempos, I don't think so, not once you're past the beginner stage.
- Is the B/C more versatile and easier to play than the C#/D?
I haven't really found that to be the case. Some things are trickier in one system than the other -- the key of G doesn't lie as nicely under the fingers on the C#/D keyboard, for example. On the other hand, on the B/C the A scale is almost all on the pull, so you have to use your air button with extreme cleverness. One thing I do like about the B/C is that with all the outside-row notes, crossing fingers over for shifts is a breeze. I had to get comfortable with shifting on the same row when I switched to C#/D. It still feels slightly clunky to me.
- Does the C#/D offer more opportunities to play bass/chords/harmonies in D major and G major tunes?
YES. Definitely. That's one of the reasons I switched over to C#/D. It's a lot easier to figure harmonies and chords out. And the basses make a lot more sense too -- each pair of bass buttons gives you I and V chords in a particular key.
