For me, the main differences between the melodeon/button accordeon are these:

* melodeons are diatonic & those in D/G & G/C have the same fingering systems for playing.

* button accordeons are chromatic & those in B/C, B/C/C# have a differing fingering system of playing compared to melodeons.

I have a D/G melodeon & a B/C button accordeon.

To play a tune in D major on melodeon, the scale is arranged with bottom D on the push, whereas on button accordeon it's on the pull.