Choosing the best approach for data selection in a datapath
A tristate bus IS a multiplexer, so there really isn't much difference.

A multiplexer is basically a layer of AND gates followed by a layer of OR gates.

With a tristate bus, you're eliminating the OR gates, but you're replacing the AND gates with the arguably more complex tristate buffers.

In most FPGAs, you can't actually have internal tristate buses. If you attempt to use them, the synthesis tool will either throw an error, or convert them to the corresponding multiplexer logic.