I would prefer it to be closer to the driver. But it depends on what you plan on using it for.
For a track car, the general concept is the on-board extinguisher is there to give you enough time to GET THE HECK OUT OF THE CAR. But for any real fire, a little bottle isn't gonna do too much good. Secondly, you gotta be able to reach it quick. Mounted under the passenger seat is not exactly easy access in my opinion. Center tunnel, or driver side long.
For a street car? Well, anywhere that's not in jeopardy of getting accidentally discharged by an errant foot would be best. But make sure you anchor it well. That fire bottle can become a significant projectile in the event of an accident. Not that it's a pressurized gas bottle (it's not that high pressure), just that it's a big heavy hunk of metal that could clonk you one good if it breaks out of the mount.
I agree though, the dry chem are great for putting out all sorts of fires, but the chemical will wreak havoc with aluminum and electrical wiring on a car. Corrosive stuff that is. Halon, and other displacing gases, are great because they're clean. But they really only work in enclosed spaces. If there's even the slightest breeze, it'll blow the Halon away in no time. Halon (and others) only works when it can fully displace the oxygen, leaving none for the fire (or you - so get the heck out of there). It doesn't smother and cover like dry chem, and it doesn't cool like CO2.
So if it's just for a get-the-heck-out-of-the-car use, I
might go Halon, and just take a deep breath before I pull that pin. Not gonna stop that fire altogether unless it's a pretty tiny one though. Dry chem will kill a small fire better, but it's a mess to clean up afterwards. Still, better than burning all the way to the ground any day...
I wonder if you can get cold fire bottles in regular fire bottle sizes...
That's the stuff CHAMP cars used to use before they were absorbed by IRL, great for A and B fires, but not for C (electrical). That would be my optimal choice I think. Relatively clean, effective, and non-corrosive.
-Josh2