The answer is not much! The M version was released a few years after and replaced the ER-5200. The reason being is that Cash Register and Credit Card interface was really taking off at that time and SAM4s replaced several models with the "M" version that allowed integrated credit card.
The register is the same cabinet, keyboard, and drawer. The display is different because the "M" version has a 2nd line to accomadate credit interface. The printer is also different and the "M" version uses dual thermal printers of 2.25" in size. I think with the exception of the interfaced payment functions the machine operates and reports exactly the same and you could even use the same manual regardless of model.
Both great machines and the M version has been a great seller over the years. Couple places to get them
SAM4s ER-5200M Cash Register
SAM4s ER5200M Cash Register