I'd encourage you to rent for this purpose. Once you get your shot, there's no variance, since the moon basically doesn't change. There is not really much possibility of getting another, different shot. If you're getting a TC solely for this purpose, that shot is going to cost your $400. And it won't be any different than the shot that all the rest of us got.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!