Grab the list from the servers
rpm -qa --queryformat='%{NAME}-%{VERSION}\n' |sort > server1.txt rpm -qa --queryformat='%{NAME}-%{VERSION}\n' |sort > server2.txt
Copy the files over to where you want to compare, probably one of the servers in question.
for R in $(cat server1.txt) ; do grep $R server2.txt >/dev/null || echo $R ; done