I think it depends on what you can do with any particular system. For quite a while, the B/C box was regarded as standard, and people said that the D/G box wasn't suitable, but I have read somewhere recently that the D/G used to be the box to use. And there are some who use the one-row box. People are now using C/C#, C#/D, D/D#. And then there are the three-row jobs - traditionally B/C/C#, but I've come across one or two in C/C#/D, and C#/D/G is now quite the thing. Playing in A on a C#/D instrument is like playing in G on a B/C, so if you are working with a fiddler, and a lot of the tunes are in A, then a C#/D box might be preferable. It all depends on whwt you want to play, and on your particular style, I suppose. Any system of melodeon - even if you're calling it a button accordeon, has advantages and disadvantages. You pick the one that, for you, has more of the pros and fewer of the cons. Of course, the three-row with Stradella bass is more versatile, but you're getting into a different instrument: the accordion.
John Kelly.