speaking more seriously,

Git with SSH transport and public key authentication is your best choice: in this case, both Git repository and the client servers can be in the public network without much concern.

"Gitolite" is a nice software package for an easy-to-setup private Git repository.

Also with Git, you can easily track the changes and roll back quickly if necessary. You can also set up hooks which will notify the client servers that the new update is available and needs to be pulled.