The 21 button, 2 row melodeon is the most commonly played in many countries accross the world, here in England we mainly play the D/G system, but in France G/C and A/D are preferred and in Germany C/F is the preferred key. The main point to bear in mind is that these instruments are identical in build to the Irish (chromatic) system but are vastly different to play.
The only main difference between keyboard types apart from the key is whether the instrument has accidental buttons on the two buttons nearest the chin or whether it has low notes in the standard melodeon scale. Certain instruments have 23 buttons to extend the scale.
A system of 2 row melodeon with 12 unisonoric basses (i.e. the bass buttons play the same note on the push and pull). Another peculiarity is that these instruments have 4 accidentals instead of 2. Usually tuned in C/F and with a musette sound - I have transposed it to D/G as well.
As I said, although these look identical to diatonic melodeons, they are tuned a semitone between the rows (B/C, C/C# etc) rather than the fifth you get on melodeons. It is technically possibly to play a chromatic two- row in any key but some are easier than others. The main difference between the layouts is to do with the bass configurations which vary - many B/C players do not use the basses.