To start, lets take a look at each item.
Banner of Command:
Stats: +10% cooldown reduction +60 ability power +200 health +20 magic
resistance Aura UNIQUE – LEGION: Grants nearby allies +20 magic
resistance and +75% base health regeneration Active UNIQUE – PROMOTE:
Greatly increases the power of a lane minion and grants it immunity to
magic damage. 120 second cooldown (1200 range).
Reason to pick it:
With the presumption that you have either one or the other, Banner of Command provides stats that would be used better on Soraka than ZZ rot would. +200 HP and the aura give your team additional healing and allows you to be more effective at saving team members rather than topping them up constantly. The magic resist as well would be much more useful against primarily AP comps.
Remember however, the minion summoned is a single unit.
Stats +60 armor
+60 magic resistance
+100% base health regeneration Passive UNIQUE - POINT RUNNER: Grants +30% bonus movement speed that builds up over 2 seconds when near turrets (including fallen turrets) or Void Gates. Active Unique
Active: Spawns a Void Gate at target location for 150 seconds. Every 4
seconds the gate creates a volatile Voidspawn that travels down the
nearest lane, attacking enemy minions and exploding when attacking
structures, but ignoring champions and void targets. 150 second
cooldown. The first and every fourth Voidspawn gain damage equal to
100% of your armor + 100% of your magic resistance.
Reason to pick it:
The resistances are much more powerful and give you regen which you need to keep topped up, However you shouldn't be taking damage as back line. The point runner passive also gives you(lets say as soraka) more speed when you're kiting back with your team, or simply running. The advantage of zz-rot is that it doesn't simply enpower a single unit. What i mean by that is, it creates several units, and as a result, might not be great against a team with good waveclear, as opposed to the enpowered minion which would take more time from an enemy who went off to another lane to deal with the wave.
My primary reason for considering this item would be if you were ahead and sieging the enemy base as "exploding on enemy strucutres) can do tons of good damage. Picking banner of command would give your team additional pressure during the mid game where rotations are significant. The additonal lane pressure could win a teamfight easily. Within the close proximity of the enemy base, the enchanced minion is much less effective.
Considerations with "mostly Soraka".
If you're playing soraka, you should know that you reasonably can't 1v1 anyone on the enemy team. Your abilities are better used with teammates. Zzrot from a mid game perspective has limited range, and requires you to go to that lane. As opposed to banner, where you can simply pop it after you recall. Because of that, for you, i would pick banner of command. The inability of soraka to roam safely alone is simply too risky for ZZ-rot to be worth it.
Banner of command if you're going Soraka.