The twin-headlight XTZ660 was the last-gasp development of the carbed Tenere line. It had a liquid-cooled motor and five valves. I haven't ridden one but the general consensus seems to be that it was a bit top heavy, under-powered and lacking in suspension performance when launched about 20 years ago.

The XT660Z is fuel-injected for performance, emissions and economy, has a modern motor from the proven XT660R, up to date styling and reasonably good suspension that is easily up to serious off road riding.