Compare RPM packages on two servers?

[floydrodery]

Problem:
Two servers running:

"Server 1" has "x" RPM's installed, "Server 2" has "x" RPM's installed, how to compare the list to see what RPM's are missing from the servers?

I've compiled a list of RPM's using rpm -qa using some --queryformat options, so I now have a "master list" of RPM's from each server. How do I compare to the two master lists to see/document which server has additional packages installed?

Things I've Tried:
I've messed around with the diff {options} command, based on the output files I've created, but the output is pretty messy and not always accurate, at least how I'm interpreting it.

Anyone have any suggestions on a clean and accurate way to accomplish this?