Yes, one is Allium cernuum and one is Allium stellatum. You can Google images or descriptions on how to tell them apart. I bet it's a subtle difference. I think the nodding onion really is supposed to have more of a drooping rather than upright flower. Wouldn't be surprised if they cross-pollinate, creating mixes even harder to tell apart.

Nodding onion definately does - that's what I have :)

-Marie

The two that I brought home from the swap last fall are the only ones blooming this season, and they are definitely upright, so they must be prairie onions, or at least a cross that includes some of that variety. The pictures I found online also showed the nodding onions as a kind of pinkish color and mine are pure white. Maybe that can vary as well, but the pink seemed pretty consistant for the nodding onions in all the photos I could find.