If you're going to the Strip and they take you through the Tunnel, just don't pay the fare. Ask for the police to be called. The cab driver won't want the police to get involved because he will get fined for longhauling. In most instances, you can settle for paying $15, the normal fare it should cost to get to the Strip, and the cab driver will have to make up the difference.
The Tunnel is only better if you're going downtown, when the fare difference is a few bucks, but you get there much quicker. So if you're staying downtown, you want the cab driver to take the tunnel, especially in rush hour.