They don't give you the same information - ammeter tells you something is wrong in the charging system, Voltmeter tells you the result after it's too late to fix it before you are stuck (glorified idiot light - yes I have opinions). I always have both because an undramatic over or undercharge issue might not be spotted on the ammeter over a long time, but will result in an under or over charged battery that shows up on the voltmeter. In a race car you can wire an aftermarket ammeter and voltmeter without the worries associated with worn out, unmaintained stock systems
They are cheap insurance...
