chicharrones
needs more ammo
For safety reasons I would choose the GP 100. I'm assuming a car gun is going to be loaded and more or less loose in the center consul, under the seat or in a locked box of some sort. The Glocks short light trigger is way to easy to accidentally fire when grabbed in a hurry.
A car mounted holster is the way to go in my opinion. The gun stays where its at so you can expect the grip to be in the same place every time. A Galco Matrix paddle holster can be bolted into a out of sight place and will hold the gun in place very solidly.
Plus, the Glocks can have the NY1 or NY2 trigger spring installed which will increase trigger pull weight comparable to a double action revolver. I like the NY1 trigger spring myself which Glock claims to raise the trigger weight to 11 lbs. as long as no other trigger parts are changed out. For only $2.00 in cost, too.
As an owner of a GP100 and a Glock, the Glock is my choice for a car gun.