I think I'd move up to the Hydra 26s for a 90 gallon tank...maybe 2 (minimum) or 3 of them depending on how long the tank is. I think 2 Hydra 26s would suffice for a 72 gallon, again depending on how long it is. In my humble opinion, potential for overkill is better than just meeting the need ... for a host of reasons. I don't think the small yet mighty Prime is quite up to the task for the two tank volumes above ... well maybe 6 of them LOL!
Having multiple lights, with more capability than you need will: distribute light better (fewer shadows), and lengthen the already crazy long life of the LEDs because you can run the at, say 60%, instead of 100% ... make sense?
My little reef is only 34 gallons and one Prime is enough to keep things happy and growing. But, already on my wish list, is a second Prime to accomplish the second paragraph above.
Another factor in choosing is water depth. My rules of thumb are:
Longer tank = more lights ( distribution )
Deeper tank = stronger lights ( penetration )
In closing, if you have more illumination than you need, you can always throttle back (with LEDs). If you don't have enough illumination, it's more difficult ($$$) to remedy.
So, those are my wholly unqualified thoughts ... you'll find as many opinions out there as there are words in this reply. Part of the fun is figuring it all out, right?
PS: There are Hydra 52s out there as well:
1 cluster (or puck) = Prime
2 clusters = Hydra 26
4 clusters = Hydra 52 (you may be able to tan with this one ... not sure!)
... just sayin'
