I agree with the two first paragraphs of RobertJSawyer.
I have an Aura HD and considered getting an Aura, but ended up asking for a refund. The size and feel of the Aura are great, the resolution and contrast of the Aura HD are the best.
The main Aura HD drawback, for me, is the slippery back but one can get used to (me) or buy a cover or something.
Have fun with your e-readers