I like the volt meter better because it is a diagnostic tool I understand. Amp meters tell you if the system is charging or not. I want to know more.
Watching the voltage as you start the vehicle gives you a good indication of the starting side of the system too. Voltage drops too low you have a problem.
Once running, if it says 14 volts, you are charging. Much more you've got a problem. Much less you are not charging.
If the motor is not running, you should see 12 volts, less you are in trouble, either discharged or battery is failing or maybe a short or load of some sort.
Volt meter helps me a lot more than an amp meter. But that's just me.
