Personally - and it is a matter of taste - regardless of their basic sound quality, the built in mics of the M10 being close-set omnis have such poor stereo imaging that I don't remember ever using them. The A10 cardioids when spread out have a good stereo image in comparison. But maybe lacking a bit at the low frequency end. However, that is easier to compensate with eq, compared with trying to compensate for the wrong polar pattern.
I must admit that when pairing the M10 with other mics, like the Roland CS binaural, I had results that I liked better, I have pretty decent microphones, like the Schoeps MK4, so I can appreciate the difference

.
With the internal mics is more of a in the spur of the moment thing, and in general I like the overall functionality of the M10.
I had a look at the A10 and - besides some design choices which I'm not a big fan of - it's also out of production, so not the easiest to source.
At the same time I have to factor in that a used M10 is going to be an oldish device, with possible failures down the line and no repairs available.
With no clear successor in that segment of the market it's not immediately easy to pick a new (or used) recorder.