I'm sure the difference which is significant to my ear is that ability to pull a bluesy 7th .
On my guitar it feels right to play the following chord progression...Em- Am- Am7 passing chord - then B7...and as an improvisation sometimes I drop in a 7th. passing chord on the Em.
I wonder also if the action of the 3 row buttons allows for slurs...well, slurs as best you can on a button accordion...Leon does them and that further adds to the bluesy feel.
there are more possible slurs than on a one row
just because there are more notes in the same direction
though to get any new slurs you have to get off the main row and find them
that flat III - major III thing i was talking about, is a slur
a half step actually
the smaller they are, the more 'fretless' they sound, and slur-ish