I pre-ordered the Fire HD 7" over the Nexus 7 because I'm a Prime member and want to take advantage of what Amazon has to offer.
I really like the Nexus 7 but the bottom line is that the Nexus 7 is the exact same thing as my phone, with a bigger screen and no "phone". Any app or function or cool thing I want on the Nexus 7 I can do with my phone.
The Fire HD will give me access to Amazon's Instant video, which the Nexus and my phone don't. Oh, and double the storage for the same price is nice too...along with the stereo speakers.