I have the Aura HD and bought the Aura on a whim with the idea to sell the Aura HD later. In the end, I didn't keep the Aura and got a refund for the reasons pointed by ErikG (granted the screen of this unit wasn't perfect, but it wasn't only that).
Even now I don't regret this decision and I read happily on the Aura HD every evening
One point though, I also have a Mini which is the ereader I take out and about. If I didn't have it, I think I would go for the Aura instead of the Aura HD due to the smaller size.