[Rastapus]

I would put it into the display with the plants. Reason being, I would assume that you are already doing large regular water changes on the grow out tank where perhaps the planted tank less so. What is the flow rate through the UV unit you have?

 

[Rastapus]

Assuming the bulb is reasonably new, that size of unit on a 46 is adequate for parasite control. Usually with a grow out tank to encourage growth, water changes would be performed very often. With that in mind I would run the unit on the planted aquarium as the water quality in the grow out would be higher.