Hey Sam,
Since you are located in PA, you will be dealing with freezing temps. If you elect to go with plastic (as I did for our project, also in a freeze prone region), use double wall for the first section feeding into the spillway and double wall for the last section at the outlet below the dam. The smooth bore of the double wall will shed water with the potential of freezing, thereby minimizing the possibility of an ice dam at the areas most exposed to the freezing temps.
The cut out section of the pipe in Zep's spillway will increase the volume of water able to enter the pipe during extreme outflows and minimize potential for vortex.
