Closed Linux OS = Bad. Nokia's Open Linux = Better
The makers of the Archos have given the device a closed version of linux. You can not develop applications for it, so you simply won't find any of the common applications you would expect ... unless Archos makes it and slaps a price tag on it.
The Nokia N810 is far more flexible. the Maemo OS is open, hundreds of applications are available from people around the world for free ... although admittedly many of them should NOT be coding ... but if you don't like it, you can build something yourself, unlike with the Archos.
... For example, Mobipocket ... not available for either device. But thanks to the open source Garnet Virtual Machine, with some effort you can run Mobipocket for Palm on your Nokia N810, while the Archos owners keep begging Mobipocket for a version ... that Mobipocket can't make due to the closed Archos system.
The Nokia N810 has similar battery life, a good screen if smaller, is much lighter and comfortably fits in a coat pocket.