Short version: Off body carry sucks.
Without going in to a lot of detail, I've spent a lot of years in a very busy Homicide office and would not recommend off body carry. It is a very good way to get shot, disarmed, or disarmed and shot. The trouble is you are generally in a reactive state as a concealed carrier and must access your weapon while under some level of direct observation by a person or persons with ill intent. People reaching for things get shot. On the nonb-bad guy side, it's also likely to cause you to be removed from your vehicle and separated if you get pulled over by police. Depending on time and context, you may find yourself at gunpoint. I never do anything wrong, you say, which is fine until your vehicle matches the description of a robbery suspect's vehicle a few blocks over...Think both odds and stakes.
On body carry allows you to feign compliance and draw in a more disguised manner. It allows you to step out from the vehicle and still be armed. It is not visible from outside of the vehicle. Imagine this: Stopped at a red light, panhandler turns carjacker and has a firearm pointed at you through the passenger side window. People say they'd drive off, but then you're at his decision to fire or not since you can't get away faster than he can shoot. Can you access that doo-dad before eating rounds? Or is it better to feign compliance, exit the vehicle so you have cover, and then deal with the problem? I know which has resulted in corpses in the seat more often.
Find a holster you can draw from while seat belted in and carry on body. If you aren't familiar with Ernest Langdon, look into his concealed carry program and 4 step draw, which once ingrained allows you to easily draw and clear the steering wheel while not flagging yourself. This is important, as I've seen many folks get excited and put a round through their leg or foot getting on the trigger early during their event. It's a real lousy day when you shoot yourself then get shot by the bad guy as well.