Is there any difference between the assembly instructions of ARM MCUs from two different corporations?
I think the correct thing to say is that for a given architecture, such as the ARMv7-M architecture of the Cortex-M3 core, the instruction set is the same for all processors. However, the behavior of some instructions may vary because of implementation-defined (i.e. optional) functionality in the processor. Instructions that try to access optional capabilities that are not implemented in a particular processor may cause exceptions.

To find the features that may be implementation defined, search the appropriate ARM Architecture Reference Manual for IMPLEMENTATION, in all capitals.