Which guage or both ?

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, resulting in several fewer things that can go wrong. Many a Mopar ammeter has been burned up due to short circuits elsewhere in the car, not to mention that any bad connections or faults within the ignition switch, the ammeter or in the instrument panel will result in a voltage drop to the coil. With a voltmeter, all of the car's load current (except the ignition current, which goes through the ignition switch only) stays in the engine compartment where it belongs.

A voltmeter will give you just as much information regarding the charging state of the alternator and battery as an ammeter will if you know what the voltmeter reads when a good battery is charging, versus what it reads with a bad battery or when the alternator is not charging the battery.

Ammeters are 20th century technology. We're in the 21st century now. That's why all modern cars use a voltmeter instead.