Nexux 7 vs. Kindle Fire HD

My wife currently has a Nook Tablet that she originally got for reading and to watch some video from time to time, but now she is looking to make a change since she is doing a lot more and the app selection is pretty thin with the Nook.

We are looking at a Nexus 7 or a Kindle Fire HD, and I am hoping for some input to what is better. She, as do I, both have android phones so there is familiarity there if we go with the Nexus. My wife reads a lot and from what I have looked online one can add the nook app onto either device, so that allows us to not lose out on books already purchased.

We have an Amazon Prime account since my wife is a student and we get it at $40 a year, and I know that includes free books (plus vide we already use too) for being a Prime member, plus it mentions able to get books from local libraries. We also have all of our music with Amazon music player.

We also have android phones and music is there too, so we wouldn't be missing out on that stuff.

I guess the most important thing is how it looks when reading. Because video dimensions of being HD are about the same as are the apps, from what I can tell.