Got some answers from Milwaukee customer service - I'll post here in case anyone else is looking for the same info:
M12 - copper up to 1.25"
M12 - BIP/IPS/galvanized up to 3/4"
M18 - 1,800 more PSI than the M12
M18 - 8,000 more clamps between servicing
So, if you need to work with copper over 1.25" and or iron/galvanized pipe over 3/4", you will need to go with the M18.