I choose to take the ar when I go. It won't be an immediate access weapon, but it will be there. If things go bad, I can make it ready and not give a crap about laws. I choose it over a shotgun for several reasons.
1) I'm envisioning an urban/suburban setting more than a rural one.
2) I'm planning mostly for riots, large crowds, etc. and want increased capacity and speed.
3) I want increased accuracy at moderate ranges to keep crowds at bay farther.
4) Although not very comfortable, it is wearable (single point sling with stock collapsed) while I drive.
If I were located in a more rural setting where wildlife and/or wilderness survival was a priority, I would probably opt for the shotgun.
With the above said, what I really want is a Kel-tec sub2000 9mm carbine. I really like the idea of being able to fold it up in a small inconspicuous package, use the same mags and ammo as my pistols, and still have increased range and accuracy. It would probably become my primary truck gun, and the ar would only go if I was going somewhere with a known history of being dangerous (i.e. Memphis).
Also, briefly continuing the laws drift, a loaded magazine is considered a loaded weapon in some places, California I think.