Nexus 7 would be my recommendation for you. But, please wait until after the 29th of this month to buy one - as there is an announcement and the pricing may change in your favor.
The Nexus 7 is the same price range as the Kindle Fire, but has better hardware. It also runs Android, whereas the Kindle Fire runs the Amazon version of Android. This means that it is slightly different to use and more importantly - you can only download apps for the Kindle Fire from the Amazon App store. The Amazon App store is a fraction of the Google Play Store as Amazon must approve all apps for the Fire and appear to be very selective.
That said, if you are a big consumer of Amazon, the Kindle Fire could be for you. It has great integration with Amazon Prime to watch movies, Amazon purchasing, etc. However, if you simple use the Kindle Fire to read books, you can download the Kindle Reader app on to a Nexus 7 and read all of your books on a Nexus.
Overall, for the same price (give or take $20 or so generally), you get a more open and true version of Android and more available apps with the Nexus 7.