I've had both for a few months, and if I were to pick one, I'd probably take the N8, mostly because of the great camera. The main advantage of the E7 is the nice screen. The Symbian Anna update brings a decent portrait keyboard.
(Based on download stats for my Ovi Store app, the N8 is definitely the most popular, followed by the C7, so I'd say it will probably see more updates for longer than the E7, but that's just a personal opinion).