Which guage or both ?

Voltmeters and ammeters each have benefits and drawbacks. I prefer an ammeter over a voltmeter, because it gives an instantaneous, real-time, right-now reading of what is happening (is power going into the battery or coming out of the battery?). A voltmeter gives a more general idea of the condition of the electrical system over time. But gauge and installation quality also factor in; I'd prefer a good quality voltmeter installed in a good and durable fashion over a poor quality ammeter installed in a problematic way.

I would not want both gauges in any one car; too much distraction.