amp meter shows you net charge current.
if you are in a slight discharge situation.. you will know it.
if you have a load or short ... at shutoff you will know it.
if you have a battery with a cell going bad, demanding more charge current, or needing high current for indefinate time during running... you will know it.
A voltmeter shows voltage.
most of the frightened volt meters i see are using an 18v scale.. and the difference between a static battery charge of 12.6 and a 'charge' rate of 14.4 ( 6.3 / 7.2 ) may be REAL hard to tell on a jittery gauge bouncing around a field.. that means you could be running off battery.. and as we know.. a lead acid battery that gets to 90% of static voltage is pretty much dead and useless... well.. again.. that's hard to gauge on a volt meter, unless it's a nice one with numbers on the graduated scale.. and not just a handfull of lines and an 8-12-18 scale... thus you could shut down then not be able to restart.. and unless you paid REAL close attention.. you would not know it til the solenoid clicked. On a ammeter all you have to see is which side of the meter the needle deflects too.. if negative you immediatley know there is an issue and you have to stop. if way + for too long than normal, then you know you have to fix it...
if I could only have 1? an ammeter.
if I could have both? sure.
I'd also take an ammeter over a charge idiot lamp as well.
I know some people here will take a volt meter, as they are used to working on cars and modern auto's.. etc.
when's the last time you saw tractors on the highway, and not tied down to a trailer. :)