I have the 605 (4GB with sd card slot) but am just about to receive the zune.
the archos is fabulous for recording TV and browsing the web (well, for a non-IE/FF browser anyway). However its a bit bulky for music and I worry about carrying something around with such a beautiful screen just to listen to music.
The Zune is something different, 80gb hard disk, much smaler then archos, glass screen, metal back and better sound quality.
It really depends whether you are more interested in video/web or music.
If you want to buy my 605 cheap (with TV recording/charging base station) i am open to offers! (I find I don't use the web browsing since I bought my asus eee pc 900)