Either is fine.
PVC will be cheaper
The box will take up more room in and outside the aquarium.
But its really in the setup on how well it will work.
Worst case scenario is power outage right?
1. Siphon will continue until tank water level is below the overflow box so siphon breaks and your sump contains the 20 gallons or whatever depending on your didplay size
then
2. Siphon breaks and pump comes back on when power is restored. The pump is separated from the main sump so only a few gallons are pumped back until the chamber is below the pumps intake and pump burns up
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