Big difference between tequila and mezcal (sometimes spelled mescal) - while they are distilled from the same agave plant, tequila's name is a 'controlled origin' name; only agave plants grown in the Tequila region near Guadalajara can be used in tequila production. Mezcal has a coarser taste than tequila, and Monte Alban's pickled
gusano (worm) is nothing more than a gimmick.
Excellent site on tequila and mezcal:
http://www.ianchadwick.com/tequila/