Which guage or both ?

Bill Crowell said
But Dan, you failed to tell grassy the primary advantage of a voltmeter over an ammeter: a voltmeter doesn't require running all the car's battery load current through the ignition switch and instrument panel

...neither does an ammeter. See for example how Chrysler set up the ammeter in '76 A-bodies and most of their other '71-up passenger cars.

Ammeters are 20th century technology.

My cars were all made in the 20th century. I'll upgrade various systems and components as I see fit, but I'm keeping my (upgraded) ammeters. If I wanted a 21st-century car, I'd go buy a new Subaru or something.