On the spillway question, always go with an open channel spillway. That will flow WAY more than you'll ever get a normal pipe to flow. If you think about it, even if you submerge the inlet of a 24" diameter culvert, it will likely be flowing around half full at the culvert outlet. That is essentially the same flow as a 24" wide earthen spillway, which is very very small. Build a wide open channel spillway and be better off.
I agree with the more water statement, BUT, if it's an earthen spillway that has water going thru it constantly, there's a good possibility that it will slowly erode over time, and when a big storm comes, it could cause considerable washout, even to the point where the dam could fail.
Like ewest said, a siphon system for a primiary. I've not seen a siphon system when activated run with anything other than a completely full pipe.