DUMB question, but I got to know???

[sam's 72]

I am going to replece my factory gauges with after market gauges, going to use Autometer. I been looking at the sport comp series, on the amp gauge, should I go with the amp gauge, or go with the volt gauge? What the difference between these two? And would it require wiring it up different if I go with the volt meter? Which one is better Amp, or volt? one more thing, which size should I go with 2 1/16' or the 2 5/8' gauge? Going to mount these gauges in the hole's that the factory gauges are in now. Going to leave the original speed o, and feul gauge, going to install a tach in the middle hole.
Sam