EFI ECM: which would you choose and why?

A mass flow system is the best method, especially for a generic retrofit. The computer doesn't need to know anything about the engine displacement, valve/cam efficiency, or even rpm to determine what fuel is needed to match the air flow, hence little or no tuning required. I am sure it does tap into an rpm signal as a trigger for the injectors, but doesn't need it to determine flow rate. The early ones (GM?) were problematic since the sensor would fail, call for too much fuel (or too little?), and melt the catalytic converter. However, I expect the newer ones are more reliable.