The 850 is black and the 800 is silver
The 850 has softer backlit buttons while the 800 has hard buttons.
The 850 has a brighter more readable backlight.
The 850 has a newer arrow pad with a dedicated select button while the 800 pad requires you to center the pad and press to do a select.
Otherwise the software seems identical and the capablities seem identical.

(Maybe the 850 can talk to some newer MRF units the 800 can't).

I like both with a slight nod to the 850.
An 800 should be priced well below the 850 due to age.