ERN miners should be a top priority in the early game.
ERN refinery doubles greenar production, which can be critical if you need early beacons, platorms, or are simply aiming for a totem speedrun. For increasing greenar production an ERN refinery is generally better than ERNing Mine Production in the ERN portal in either short runs or maps where a single refinery can keep up with greenar crystal spawns.
ERN cannons are extremely good if you would otherwise need multiple cannons very early, but should be switched elsewhere once you stabilize.
Portal: Even after the ERN Portal is built and an ERN is slotted into it, the effect takes 2 minutes to charge up to its maximum effect. Because of this an ERN Portal is often not used in vey short runs.
Portal: In general: Energy production » Weapon range/firing rate » Move speed>build speed, however different maps may have different priorities. Mine production is so map dependent that it is not listed above; in some maps it is practically required and in others it is completely useless.
Increased range on a beacon/pylon can really make a difference in some maps, but is typically not useful.
Don't be afraid to move them around once the problems they were solving are dealt with.