Todd,
You should also probably avoid corrugated plastic tubing (single or double walled) for two reasons. One is that is actually bouyant, and you need to be careful about ensuring the inlet is held down by a lot of rock covering it. However the bigger issue is that NRCS is starting to see issues with the double walled pipe forming cracks down the length or around the circumference. In SD they recommend schedule 40 sewer pipe.
Tim is exactly right about the reason for using a drop inlet on the tubing. Our bigger dam has a 5 ft diameter rising flowing into a 2 ft horizontal pipe. When 6 inches of water is running into the inlet, the starting part of the 2 ft tube is completely full, but by the time it drops 15 ft to the outlet it is only running 1/4 full (but very fast).
The diameter of your tube is determined by the watershed area, topography, vegetative cover, and 25 or 50 year precipitation events. A 6 inch tube is small and subject to plugging, so I'd recommend at least a 12 inch tube. The pipe won't cost that much more, and installation will be the same.
