Last Thursday I bought a 5800 and now I have both devices as well.
Here my impression:

..::N810::..

SPECS:
- Screen 800x480 / 4"
- ARM processor 400 MHz
- MAEMO OS2008 Debian like Linux
- 128MB memory, 2GB internal storage (of which 128MB swap space), max 32GB external MiniSD storage

PRO:
- Open source system full fledged Linux
- No hassles with certificates
- Large screen for a pocket device
- Browser handles virtually all sites properly
- Lots of standard access to and from device (SSH, SFTP, VNC) as it is Linux
- Lots of freeware applications (the only paid application is the excellent Wayfinder Map)
- Can be used as webserver (Apache / Lighttpd with PHP 5)
- No bloated Windows apps (such as PC Suite, Maps Loader) needed for Updates and syncing, it can update itself like a real Linux computer.


CON:
- Somewhat heavy (200g)
- No office suite
- No phone (other than Voip client) but can use a GPRS/UMTS with Bluetooth as a modem
- Some touchscreen elements do not work properly (e.g. you have to click 10x before checkmarks / radio buttons respond)
- Poor GPS



..::5800::..

SPECS:
- Screen 640x360 / 3.5"
- ARM processor 369 MHz
- Symbian S60v5
- 80MB memory (of which 40 MB user memory), max 32GB external MicroSD storage

PRO:
- Smaller pocket size
- Large screen for a phone but smaller than N810
- Touchscreen slightly better than N810 (checkmarks work and feedback by vibrating)
- Lots of nice skins available for free

CON:
- No virtual memory which results programs will be killed with a silly 'memory full' message (although this is rather rare compared to older phones)
- Browser not complete yet (cookies do not work on local pages, some sites are not rendered properly, and user agent cannot be changed, which limits some sites to see only limited mobile content)

What BOTH have:
- Builtin GPS (N810 is poor, 5800 unknown as I use a Bluetooth GPS)
- Touchscreen
- Possible for guided navigation (paid, about $100 extra) and free mapper and other GPS related applications and both can use Bluetooth GPS
- Use as VoIP client (N810:Skype, Both:Fring, etc.)
- Use as media player
- Use as web browser although the 5800's browser is somewhat limited
- Use with instant messaging and mailing



Both devices together cost less or at most the same as a single N97 but have lots more functionality. The price is for both about the same, about EUR 320 in Europe (which might equal $250-$300 in the US).