they are both similar phones.
The main difference is that the e7 has a bigger screen and slide out qwerty keyboard. The n8 has better camera, the ability to add up to 32gb memory card, and has a better package of features overall.
I have the n8 because i wanted the better camera and extra memory, its an amazing device and if i didnt have it, i would have the e7.
Check out the official
specifications and decide for
yourself.
N8
http://www.nokia.com.au/find-products/all-phones/nokia-n8/specifications
E7
http://www.nokia.com.au/find-products/all-phones/nokia-e7-00/specifications