The spillway will be for emergency draw-down only.  Under normal conditions the flow is regulated through a spillway channel at one end of the dam.  This siphon will only be operated once a year and in emergencies if the spillway channel is overwhelmed.  My two main design considerations are vandalism resistance and zero leakage, but I also care about torque & ease of operation.  I am considering a gate valve and a butterfly valve. Maybe I'm missing something but I don't see why one would be better than the other, except it seems the butterfly valve would give quicker operation.
Assuming you plan to use line size valve. B/F valves have lower pressure drop (Higher Cv)and with constant head will have greater flow rate. Gate valves are generally more rugged. Soft (rubber) seated B/F valves and wedge gate valves will both give bubble-tight shut off. B/F easier and quicker to actuate. There are very secure key interlocks to prevent unauthorized operation. For low pressure drop applications you could use valve with direct lever operation and padlock.

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