I would prefer ECM. Why? Shots don't lose their potency when traveling through space (usually, certainly not missiles, shells, lasers, or most of the weaponry that AI war uses), so it would make no sense for shots to be decflected worse from close as from far. ECM makes sense: the farther you are away, the more ECM systems can jam your targeting sensors, making ships harder to hit successfully. it also fits in with "targeting systems" better Moreover, certain special buildings make more sense with ECM over deflectors (radar jammers).
Shells have a higher chance to miss the further away you are
Lasers diffuse and lose overall power over distance
Missiles, well this one requires a little "magic".
The further you are from an enemy missile launch the higher chance the ships computers have to prepare for, and therefore bat away a missile effectively.
Basically, distance does matter in two cases, and can be explained through lore on the last case. Distance affects proficiency with a weapon, even in a vacuum. Even more so with deflector shields.
Additionally a Zenith ECM booster makes no sense. Alien technology may or may not even use the same capable systems, additionally, a large part of Zenith ships are biological, making ECM almost completely illogical in this case. Afterall you can't really jam a targeting computer of a human, except by a large flash of light. In this case, we already have that, its called a laser.