My advice is to do both. Have a switch on the ignition going into the PS and have a momentary switch for the power on.
Computers crash, its a good idea to have a manual turn on/off for it.
And if you just want to move the car down the street or let someone out thats in front of you, have the switch on the ignition so you can turn it off and not have the computer boot up for the few seconds that your car will be on.
yeah i would add the switch in before the power controller... that way, off means all off- and its handy when you need to 'reset' the power logic by removing power for a few minutes. this has helped me greatly with my m2-atx... if it gets confused, or my mainboard does, just flick a switch and all gets shut off and reset.
I have had my carnetix for over 3 years and never had an issue with the Logic getting confused.
But I will agree that a switch to prevent the computer from coming on is a definite need. A lot of times you dont want your computer to boot if making short jogs to the liquor store or gas station. I also have a momentary switch wired to the power button and one to the sleep button. This give me two options for manual shutdowns. I have a high shutdown delay on the carnetix and sometimes manually shutdown or standby the PC if wanting to just park momentarily.
Recently I am using a script I wrote that will controls the shutdown sequence in various methods.
It helps automate shutdown and a lot of other things I want my carpc to do.