There are benefits to each of these types EFI system. We aren't even going to consider some of the other types of EFI systems currently available since we are only discussing V-twin engines. The choice of systems can be dependent upon the type of vehicle and its usage.
In the case of the HD Magneti-Marelli EFI system, the simplicity of using the throttle position sensor (Alpha-N) for determining engine load was probably the determining factor in using this for the first OEM EFI system.
A Speed Density EFI systems need a steady vacuum signal from the MAP sensor for the ECU to accurately determine the fuel mixture. If you have ever tried to read a vacuum gauge connected to a v-twin engine and watched the needle bounce around, you might begin to understand why a Speed Density EFI system was not the first choice for Harley-Davidson, engines with long duration cams or individual intake runners. Despite what appears to be an insurmountable problem with creating steady vacuum readings, there are a multitude of mechanical and electronic ways to "calm" the MAP reading for the ECU to use. The Delphi EMS system is able to use the manifold pressure from the HD intake, allowing it to more accurately determine the load on the engine based on the actual vacuum in the manifold rather than the position of the throttle. The Delphi unit also offers a level of sophistication and an ability to customize fuel maps in the field.