If it's mostly text, you'll probably be happier with the smaller device, for it's easy of handling. If it is a lot of graphical content, like comics or magazine style layouts, the bigger screen is much nicer.
At least that's how it breaks down for me. I have both.
Despite that, I still tend to do most of my reading on the iPad. I'll switch to the Nexus 7 if I'm reading a Kindle book in bed; but if I'm at my desk I'm more likely to use a stand, so I can keep the bigger text and dual page layout (personal preference).