[inquisitor]

Just got the information, that the "Tchibo Internet Stick", which is sold for € 50 including one month of free internet access (worth € 20) does definitely support UMTS 850 and 1900, so it generally would work on AT&T's 3G network, if it wasn't locked. But you can unlock it after leaving Germany for € 15. So the total cost for the Tchibo stick (purchase, 1 month internet, unlocking) would be € 65.

Even if the Vodafone stick "K3565" supports American 3G bands, too, it wouldn't be significantly cheaper, as you would need to purchase the stick (€ 30), buy a Tchibo SIM (€ 10) and topup € 20 for the internet flatrate, so € 60 in total. If you consider the effort to order a Vodafone stick, buy a Tchibo SIM, get the topup voucher and all the technical challenges (Vodafone stick needs to be debranded in order to use other SIM cards and then the access software "Mobile Partner" doesn't support USSD-commands, which are necessary for topups), the Tchibo Internet Stick appears to be the better choice.