Tough choice! I bought a Nexus 7 back in August and absolutely loved it, my first tablet. After using a friends ipad for browsing a few times I realised how much I preferred the bigger screen.. But the benefit of the 7 is how easy it is to take everywhere with you, it's so light and easy to use if you commute by train...
So I would say if you plan on reading books on it, using it on the move a lot and aren't too bothered by the screen size, go for the 7.
If you aren't limited for space and will rarely use the device on your travels, then you may enjoy the bigger screen on the 10, the display is incredible.
I personally haven't really missed my 7 since I got the 10!