The siphon would create more suction than figure two with the valve closed (it looks like the closed loop comes up near the pond surface), but only when in actual siphon mode. When it goes into siphon mode you have a real head for the flow, depending on the elevation diff between the pond surface and the outlet of the pipe. Say the outlet of the pipe is 4 ft below the pond level. When siphoning you have a true 4 ft head, which creates a good bit of vacuum, about 1.7 psi which is pretty strong. Of course with the figures you posted above, you'd have plenty of head, way more than 4 ft. when you opened the valves wide open, in either figure you'd have a lot of suction with the valve open. But one thing I really like about the siphon is that when the water is really flowing into the pond in a heavy rain and bringing in a lot of silt the siphon kicks in without having to turn on the valve, so the siphon is drawing with strong suction during the event (without me being there), near the bottom where most of the larger suspended particles should be. I'm leary of any pipe thru the dam, so I like the fact that the siphon can pass thru the dam at a higher water level. I'm sure either way would work fine.
