I know nothing of this "Archos" so I'll just ask questions that might help you decide. You'll have to find the answers.
What is it's web browser like? The iPod's is freak'n great, so that's something to compare.
What will your video usage be like? For me, I convert a DVD or two at a time, put it on the iPod and then remove it after I've watched it. (But leave it on the computer.) The point is, for me, that I don't really care WHAT formats are supported since I have to convert the DVD first no matter what.
What size and weight is the Archos? You can actually forget you have an iPod in your pocket, which is great. I'd consider the "how will I carry it?" question and make sure the device you pick matches your plan.
This post probaly sounds biased towards the iPod, but it's really because I don't know this other device at all. All I can tell you is that I love my iPhone (I made this whole post on it) so all I can do is tell you what I'm familiar with.