Actually, if you'll research the facts, you'll find that B&N's claim of most books is rather smoke and mirrorsish.

And as far as prices, thanks to the agency model forced on everyone by Steve Jobs, for four out of the five major publishing houses, all vendors selling a given ebook are REQUIRED BY CONTRACT to charge the same price, set by the publisher.
But to actually answer your question.....
To my mind, the most important features are the bookstore you want to buy from, and the quality of the ereader associated with the store, not how good their android app is. The Nook Color looks extremely cool and attractively-priced. But for me, the eInk Nook can't keep up with the Kindle 3, and I find eInk the most comfortable way to read text. For books with significant illustrations (of which there are very few translated into good ebook form) I actually prefer reading by using the appropriate app on my iPad! (don't beat me!).
I read on a Kindle, an iPad, and on my Android phone, so I think I can pretty well say I'm not a fanboy of any given brand or store, but I do find the Kindle 3 the most pleasant way to read text for lengthy periods of time.
Whichever ereader you decide best meets your needs, don't forget that there are independent ebookstores such as Smashwords and Baen Books (great selection of science fiction that is cheaper than Amazon or B&N).