$ ssh server-a dpkg --get-selections | grep 'binstall$' | cut -f 1 > server-a.txt
$ ssh server-b dpkg --get-selections | grep 'binstall$' | cut -f 1 > server-b.txt

$ comm -12 <(sort server-a.txt) <(sort server-b.txt) > both.txt
$ comm -23 <(sort server-a.txt) <(sort server-b.txt) > only-server-a.txt
$ comm -13 <(sort server-a.txt) <(sort server-b.txt) > only-server-b.txt

$ wc -l *.txt
  2238 both.txt
   948 only-server-a.txt
    89 only-server-b.txt
  3186 server-a.txt
  2327 server-b.txt
  8788 total

apt --installed list | tail -n+2 > that_server.lst

./compare_ubuntu_apt.pl this_server.lst that_server.lst

----------------------------------
 DIFFERENCES
----------------------------------
 @ apparmor : 2.8.95~2430-0ubuntu5.2 -> 2.8.95~2430-0ubuntu5.3
 @ apt : 1.0.1ubuntu2.8 -> 1.0.1ubuntu2.10
 @ apt-transport-https : 1.0.1ubuntu2.8 -> 1.0.1ubuntu2.10
 @ apt-utils : 1.0.1ubuntu2.8 -> 1.0.1ubuntu2.10
 @ base-files : 7.2ubuntu5.2 -> 7.2ubuntu5.3
 @ bash-completion : 1:2.1-4 -> 1:2.1-4ubuntu0.1
----------------------------------
 MISSING IN this_server.lst
----------------------------------
 + acl : 2.2.52-1
 + acpid : 1:2.0.21-1ubuntu2
 + apport : 2.14.1-0ubuntu3.11
 + apport-symptoms : 0.20
 + at : 3.1.14-1ubuntu1
 + at-spi2-core : 2.10.2.is.2.10.1-0ubuntu1
 + attr : 1:2.4.47-1ubuntu1
 + autotools-dev : 20130810.1
----------------------------------
 MISSING IN that_server.lst
----------------------------------
 - apcupsd : 3.14.10-2build1
 - apcupsd-doc : 3.14.10-2build1
 - beep : 1.3-3
 - btrfs-tools : 3.12-1
 - discover : 2.1.2-5.2ubuntu1