Aujrb, my opinion is, it's not nearly as all-around-safe for you as carrying a handgun on your PERSON all the time.
When you're out of the car, you're away from the gun, and all but helpless to respond to a lethal threat.
When you are away from the car, anyone who steals the car or even breaks into it can steal your gun without opposition.
But when the gun is on your person, it is at once accessible to you at all times, AND inaccessible to unauthorized hands.
If you live in a jurisdiction where you CAN have the gun in the car but CAN'T have it on your person, I'd go with the gun in the car because "something, sometimes" is better than "nothing, ever."
And -- it goes without saying that you want to be familiar with the laws where you are about having the gun in your car in certain places (workplace parking lots, "gun-free zones," etc.)
Moreover, be aware of the "out of sight, out of mind" factor...once you have a loaded gun in the car, be sure you don't lend the "armed vehicle" to your underage kid, or your brother-in-law who has led an exemplary life since his long-ago felony conviction, but never got his felony conviction expunged. In can get those nice people in deep trouble since they will now be in "constructive possession" of the firearm within the vehicle, even though they didn't know it was there.