How do you know the USB key is safe?
Then change "USB key" into "SD card", and make sure it's properly erased before use?
CD ROM is best.
Cannot be altered once finalised, so you can use uncompromised systems to check whether the contents of the CD ROM have been burned as per your instructions (and so you can reliably detect whether your Armory dependencies packages are definitely what they purport to be).
Also, as the OP states, these instructions will work for Debian 7 "wheezy" and Debian 8 "jessie". Ubuntu is essentially a bloated fork of Debian, targetting the consumer/business market. Not really necessary for offline Armory as such, Debian will do the job just as well.