they are completey different ecus, one plug and play the other a new loom or adapter(approx $400 from JEM efi) and much more setting up is required before you can run the engine. the motec has extra outputs allowing extra functions to be added, and full data logging can be used if it is enabled. I dont think the m4 has the required injector and ignition outputs to run a rb25 in full squencial mode, but you would need to check.
New have no idea on the price, but if you need a ecu with the outputs/features they are more than worth the money, so are you going to use the data logging, wide band lamba meter, have thrmofans etc that need controlling. what do you wont the ecu to do?