They both have their place. I mostly use a voltmeter for diagnostics. When you know how to read the nuances of the meter you're using it generally tells you about all you need to know.
As far as I'm concerned, in terms of a charge reference on a machine with an alt... the old idiot light is as good as any.
An ammeter can also give you a lot of good info tho.... very quickly show you a discharge situation if it's wired correctly, belt slip, no charge, etc. IF you know wht you're looking at.
The main thing I use an ammeter for (current clamp) is checking starter draw. For that, it's indespensible.