N8 best for the camera.
N9 better at everything else.

If you are a photo-maniac, then it would make sense you have a good quality camera (or the N8) with you all the time.
If you are just a layman, the N9's camera is *good enough* so you should upgrade.

But then again if you are a layman, don't get the N9. There's much better offerings from the Android side, where you also get a better operating system, since Harmattan is crippled on the N9 (the N950 is a different story).