I am in the process of building my siphon system now and I went with 8" and the cost was right around $2000 for the schedule 40 pipe and fittings.
I was going to go with the 10" but the guy from the NRCS said that he wouldn't, just because it is so hard to put together. I never even priced the 10", but the 8" was pricey enough.
I built an emergency spillway, so I am not real concerned about an overflow every once in a while. I have about 75 acres of watershed, so I am sure that it will happen.
Also the 8" is heavier and harder to handle than I expected.
