Veets
2
I did some tests once upon a time and MD clock has a little more jitter than OT clock. Assuming the modern clock has made it into the A4, then I would say A4 will be the better clock and therefore the master.
However MD has some MIDI machines to mess with your A4 patterns so for creativity you may want to have MD master. And if Ableton is master of MD, jitter may not be an issue so again you may want MD as master.