I agree with SoTxBoTi. An ammeter doesn't actually compare voltages...although a voltage differential will cause current to flow.
It measures the total current flowing between the battery and the alternator. If current is flowing in such a manner as to charge the battery, a charging reading will be seen. If current is flowing that is draining the battery a discharge will be indicated.
The thing that I don't like about ammeters is the fact that they require all the system current to flow thru them. This can result in wires overheating due to poor connections, added complexity and just plain old high current stuff under the dash.
I prefer volt gauges...I have them in all my vehicles except my 64 C10. I've got a feeling it's gonna get a volt gauge eventually. As was mentioned, you can tell a whole lot more by reading a fairly accurate volt gauge. You can tell the condition of the battery with a small load (essentially no load if you just switch the ignition on), the battery condition under load (observed while cranking the starter), the alternator output and the load that different accessories put on the system.
It's a pretty handy tool for troubleshooting and for warning of possible problems.
But (as was pointed out) if you're going for a totally stock look, then ya gotta have the ammeter, I spose.
Just my opinion,
