I would not use any valve to restrict drains. Drains should be wide open IMO
I'd go with an 18 and use a ball valve at the outlet to choke it back if needed.
The flow through the sump has to be fast enough to efficiently feed all of your components that rely on water being fed to them, like your skimmer, and any reactors, but can't be too fast, unless it's like a tsunami in there and you are getting a lot of micro bubbles in the DT as a result.
Slow flow through the sump is completely unnecessary.