I've posted these pics elsewhere, but...
This is the "M Series":
This is my M-A1:
As the name suggests, the M-A1's are just a revision of an existing design.
The slides, barrels and sites all seem to be pretty much the same. I'm not sure if the mags are interchangable, but I suspect they are. The barrel on mine in the picture is longer only for Canadian legal reasons. The M Series pistols that I have seen in Canada have been restricted firearms with 106mm (rather than 102mm like in the picture) barrels.
The grip has been redesigned and is supposed to be more ergonomic now. This seems to be largely cosmetic though. Both pistols have more of a "1911 style" grip angle than other competitors like Glock.
The colour has been lightened up on the new ones.
As previous posters mentioned, the A1's now have a standard accessory rail, which is a lot more practical if you are actually going to add a light, compared with the rather strange "box holes" on the M Series.
As another poster mentioned, the safety on the M Series is now optional, and all of the M-A1's I have seen in Canada have
not had the safety.