There are only one HSC and one PTO in ML1200, versus two of each in ML1500. ML1200 has no internall real-time clock (extra module only).
Their extension modules are not compatible.
The 1500 CPU is a separate piece of hardware that slides into the main unit. It may be with one or two communication ports.
The 1500 main unit may be of different types of I/O.
The 1200 is single unit, one or two ports.
1200 and 1500 main units choise is different by size and I/O type.
These are, generally speaking, all the major differences.