If you are already sure that you will buy an iPhone then you should probably stay within one OS and buy an iPad as well. All your apps will work on the iPad that you buy for the phone. Of course some people don't like reading on a backlit screen, some don't like reading on a kindle screen. Your mileage will vary on that point. Battery life is another concern, and a big plus for the iPad. The next generation iPad, and iPad mini, will be out within a few months, with less weight, better battery and higher resolution screens.

You can pay less, but then you get what you pay for as well. There are real reasons the iPad family has the sold better than any other tablet.