The choice between internal and external mics depends on the type of sound you want. Even with the exact same mic, the results are VERY different. If you want to go for a Cajun sound, it is best to have the mic on the outside. This gives a nice clean sound. But for zydeco, it is better to put the mic on the inside. This creates that cool muffled sound that we associate with zydeco greats like Keith Frank(the sound is somewhat like what a blues harp player gets).

On my D accordion, I have the option of switching between internal and external with the flip of a switch. I suppose I could even use both at the same time, but I haven't tried that. Lately, I find that I just leave the external mic at home.

-David