[Am I to assume that the only difference between a diatonic two row button accordion and a melodeon, is the melodeon is a single row instrument?]
“melodeon” is sometimes used in a specific sense to mean, a single-row instrument which is BOTH:
diatonic
AND
bisonoric
however, “melodeon” is also often used in a general sense to mean ANY button box regardless of how many rows, if it is:
bisonoric
SOME bisonoric button accordions are
diatonic…
and OTHERS are
fully chromatic
people often ignorantly or (in cases such as my own) sloppily use the term “diatonic” when what they really want to say is, bisonoric.
two-row “semitone” bisonorics, where the row keys are a half-tone apart, such as b/c, c#/d, etc, are not diatonic. they are fully chromatic, just like piano accordions and chromatic button accordions. however, PAs and CBAs are unisonoric.
with a “semitone” two-row, because it is fully chromatic, you can play in any key. you will not have basses for every key, because the basses are very limited. but you can play treble side in any key, if you have the stuffing to tough it out and learn the scales and fingerings, which are not that hard, but which are counter-intuitive and have a steep initial learning curve.
finally, semitone boxes have equal capability to play an equal number of keys in a “smooth, flowing” style (often ignorantly called “b/c style”), and equal capability to play equal number of keys in a “pumping, back-and-forth” style (often ignorantly called “c#/d style”). it’s just a question of which keys in which box tuning.
if you are dying to be “pumping, back-and-forth” style in D, A, and their relative minors, you might like C#/D. on c#/D, E major and B major will be more “smooth.” you can play those in piper “B sessions” if you are so inclined.
on b/c, you get to be “pumping back-and-forth” ish in the east clare-esque keys of C, F, a minor. you get to be “smooth and fluid” in e minor, D Major, E Major, A Major, and kind of in d minor. G, g minor, and B flat are kind of 50/50…and you can play “on the row” on your B row for those super-flat piper seshes, and teach yourself e flat, a flat, and b flat on your B row as well. (of course, you can learn those keys on your c# row of a C#/D too)…
i do notice that b/c seems to lend itself quite well to playing in ALL keys, which i really love. theoretically, c#/d can of course do the same. but you do see plenty of c#/d-ers lugging b/cs around to play “back and forth” style on when they play in flat keys. i love using only one box and sometimes playing smooth, sometimes back-and-forth.