I choose the M over FF most of the time for macro because of working distance.
Bingo. You can get the shot either way, but the smaller sensor will give you a little more room to work.
6 pages of people arguing tiny differences that make little or no difference in the real world or your final images. Sensors are so good now that there are only two categories where format seems to matter at all:
* Really low light stuff like astrophotography and astro-landscapes. FF tends to dominate here though you still see good work from APS-C.
* Wall sized landscape prints with incredible detail and zero noise/artifacts. You have to see these in a gallery to appreciate them, and they are all scanned and digitally processed LF film, or MF digital.
For everything else it matters little whether it's FF, APS-C, or 4/3. For some reason we love to pretend otherwise.
Looking over the best macros at a photo sharing site like Flickr, the key to top rated macro work is not format, but mastering focus stacking and lighting. Which camera do you choose? Whichever one appeals to your budget and desired features.