I own both, here's the problems/limitations I found with each...

n9:
* Very difficult to write with it, painful to code.
* Couldn't use it as a secure WI-FI hotspot (there's only wep).
* Couldn't use it as a USB modem on linux reliably.
* Couldn't play webm videos embedded in a html5 document.
* Couldn't find a FTP application.
* The only (3rd party) whatsapp application was unreliable.
* Couldn't watch embedded vimeo or youtube videos.
* Couldn't interact with embedded applet content on sites (like wunderground radar).
* No widgets, just an old-fashioned icon-grid as well as a multitasking pane and notifications pane.
* No inbuilt per-person aggregator for communications & subscriptions (web-feeds, social-media, email, etc.), however there is an most-in-one aggregator (notifications pane, no Google+).
* No useful OCR technology.
* 3rd party applications are mostly spammy, many are just web-portals.
* Low volume; I always missed calls in the mall.
* Noisy photos in low light.

808:
* Couldn't use google-chat.
* Couldn't select & copy text in the email client.
* Multitasking is slow & tedious.
* Inbuilt keyboard doesn't have arrow-keys (anymore, thanks Nokia), swype only lets you enter text in full-screen mode.
* Couldn't use it as a USB modem on linux reliably.
* Very difficult to write with it.
* Couldn't use it as a secure WI-FI hotspot.
* Couldn't do python scripting on it.
* Couldn't play webm videos embedded in a html5 document.
* Couldn't watch embedded vimeo or youtube videos.
* Couldn't interact with embedded applet content on sites (like wunderground radar).
* No inbuilt per-person aggregator for communications & subscriptions (web-feeds, social-media, email, etc.).
* No useful OCR technology.
* Couldn't find a FTP application.
* 3rd party applications are mostly spammy, many are just web-portals.
* Unstable; stopped checking gmail once, rebooted itself a couple of times.
* Camera would sometimes shoot black frames, had to switch settings at random to get it working.
* Couldn't hot-swap the simcard.
* Unbalanced when using with one hand; physical buttons at the bottom, heavy camera at the top.
* No lens cover, lens is right where my index finger goes when holding the phone comfortably.