For me it takes both but if it is necessary to choose personally I prefer an ammeter which indicates directly if, at the moment m, the battery is charging or discharging, that is to say if we consume more or less than what the alternator is capable of supplying.
A voltmeter provides only an indirect measurement but it has the advantage, with the engine stopped, of providing an approximate indication of the charge level of the battery.
I planned to install, where If-tp2511 has installed its voltmeter, an USB charging socket and there are models that have a built-in voltmeter
