Can someone point me to the differences between Arduino Zero & the M0 Pro?
I can't find the differences and would like to understand them before deciding on buying one or the other...
Just figured your question and I'm asking me the same
The Arduino Zero and M0 Pro are very similar, but digital pins D2 and D4 are swapped and the Arduino Zero has an additional ATN GPIO pin, (nearest to the DC power jack and next to the IOREF pin).
The two boards also use different bootloaders, but this can easily be remedied by burning the a the bootloader using the on-board EDBG chip using the Arduino IDE.
Compare the schematics 
Both can be downloaded from the respective product pages.
There seems to be a pinout difference (connection from micro to pin); I think it's on the header for D0..D7 but not 100% sure (OK, confirmed by previous post by MartinL.
I found Difference between Zero, M0, M0 Pro - Arduino Zero - Arduino Forum; not much difference except for what is mentioned above.
Was the M0 (Pro) designed by Arduino.org and the Zero by Arduino.cc ?
The M0 Pro and the Zero have a seperate serial port for debugging.
I think the MKR boards don't have that and also the M0 does not have it.
All these boards have a 3.3V SAMD21 M0+ processor.
The newest boards are the MKR boards. The MKR Zero is a nice board.