Hey estratos,
thank you very much for your reply. It's good to know that the panStamps can communicate between each other and I think it's a very interesting device which I'll keep in mind for future projects or maybe this project at a later point.
But after I read that the XBee can be used without an additional microcontroller to send "raw" data, I think I'll go for the XBee for now. As I'm totally new to wireless networks, I started to read the book "Building Wireless Sensor Networks" which explains how to use the XBee step by step. That's what I need for now and I couldn't find such step by step tutorials for the panStamps.
But like I said, after I have used the "easy" XBees and have a better understanding of wireless networks, maybe I can switch to something more advanced. But thanks for the advice and also the link to the wiki.