Which guage or both ?

Running both doesn't really make sense to me because in most cases people switch to voltmeters in order to get rid of the dangerous factory ammeter setup. The only real advantage is safety. So if you're thinking of running both for whatever reasons you might as well just run an ammeter because it will tell you everything a voltmeter does (and more). You're just wasting time an money installing a second gauge that tells you pretty much the same thing as the first.