They're the same command, someone just added the inverse because people kept mixing the command up and typing it wrong.
An easy way to find this out would have been:
minibun:~> ls -l `which apt-add-repository` lrwxrwxrwx 1 root root 18 2011-08-26 15:36 /usr/bin/apt-add-repository -> add-apt-repository
You can see that
apt-add-repository is simply a symlink to