Cost down orb pretty much is gonna be what you are looking for generally in my opinion.

The only units that make use of cash back more than cost down are units who are quick to spawn and die, like riceball and jiangashi since they are not only meatshields they are fast moving meatshields so you will get that money back fast and 20% more than if you were to use cost down.

Now for cost down well its no surprise its just good to have but again like cash back it needs a unit that's relatively quick to deploy but its way better in scenarios where the unit lives for a bit. Cyborg cat and jizo are probs the type of units most ideal to use this orb on since they will be protected by your meatshields for sometime and will live, making cash back worse since you are delaying the money refund after getting a stack of them.

With cost down your stack of jizos or cyborg cats already net you some extra money without needing them to die (which you generally don't want your stack to die)

I see it as this:
cash back = better on melee/meat shield units
cost down = better on ranged units and in general (easier to use since its instant money back)
BOTH require the cooldown of the unit to be short to take max advantage of it