For that dirty Zydeco sound internal mic. For cajun and Zydecajun a la Wayne Toups external. For external, you can go old school with the snap on Sure mic cartridge and look like Nathan Abshire or new school with a clip on mic mounted to a bracket or the bellows strap. Larry Miller sells both internal and external with instructions. Steve Riley has a neat set up with an external mic while standing next to a stage mic for the bass side when he needs it. External gives you that clear sound you need for the cajun riffs. Internal rumbles the insides of your audience. Both keep'em dancing. Hope this helps.
I've tried internal mics on all of my accordions and I just don't care for the sound. To me they sound like you're playing in a 55gal. drum inside an outhouse. Not to bust on anyone who likes that sound, I just don't. I use an Audio-Technica ATM35 with a wireless unit for my Cajun accordions, and a Microvox M305/310 for my triple rows. The ATM35 clips to a fitting I had custom made for my accordions because they are all special made for me by Junior Martin and I didn't want to drill into them. I had a metal worker perfect a kind of 'S' bracket which attaches at the screw hole of the top bellow strap fitting. I unscrew the top bellow strap, place the 'S' bracket on top of the accordion (lined up over the hole) and re-attach the bellow strap with the same screw which now holds both the strap and the bracket. Then, I simply detach the mic from one accordion and attach it to the next one I'm playing. The Microvox setup is a hard line into a four channel pre-mixer which then connects into one channel on the main mixing board. This way I have all 5 accordions equally mixed and EQ'd, then plugged into a single channel on the main board. This works very well for me. I like the external mics because (as the previous fellow said) you can get a really clean, fat tone from this, and you don't have all the noise associated with the internal mics. Now, if you're only looking for one sound then go for the internal, if you like. But, you have much more flexibility with a good external mic.