[bobH]

For the amp gauge, you are running heavier wire to it and through it... big enough wire to run the car, including headlights and whatever else you have that draws a bunch of current. And, this heavier wire has to be run from the engine compartment, through the firewall, and included with the under-dash wiring. In my opinion, safe enough with stock, properly installed and maintained wiring. But, also my opinion, if you have an aftermarket amp gauge, with homemade wiring, and if you screw up the install, there is the possibility of bad things happening.
On the other hand, if you go with a volt gauge, the wire to it can be very small (and fused small, if you like), and this wire will run the volt meter. If you screw up, and bad things try to happen, this small wire, or a fuse, can burn out, and no harm is done. So, for many (most?) modern cars, I think a 12V system, with a simple volt meter, is potentially a little more fool-proof and safer.