As Thom says 2 different Orbiter software flavours, two very different devices, it really depends on how you want to use it.
The n800 kinda fits in your pocket, I find that Orbiter causes the batteries to drain pretty quickly, so it has to stay plugged in the majority of the time. It can be tough to hit some of the buttons without using the stylus. My n800 lives on my Dining Room table, it's very small and unobtrusive.
The Archos, I've only used one (an Archos 7 Internet Tablet) for about 5 minutes, I liked it for the most part. It ran Android (lots of apps), it was snappy and the screensize is bigger, which is better for 'coffee table' Orbiter use (flimsy built-in stand). I have not had a chance to try Orbiter or TouchOrbiter on an Archos itself. The Archos 7 Home Tablet is the same size with less speed/graphics capability than the Internet Tablet and would be more than enough to run Orbiter alone.
It comes down to what you want to do with your device and how you want to use it.