Use both. I put a shunt behind the ammeter to avoid overloading it. A piece of a stainless steel hose clamp in a "U" shape between the ammeter terminals works well. The real question is WHY anybody would want to use an 80 or 100 amp alternator in the first place? P.T. Barnum must be in the electrical parts business these days!

To properly monitor what's going on in the charging system it would also be beneficial to have an onboard hydrometer to monitor the specific gravity of the battery acid, in addition to knowing the charging voltage and amperage, and enough experience at repairing charging systems to correctly interpret those three readings.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!