On the car battery thing - I think that a car battery (if disconnected from the car) would be a cleaner 12v supply than even a good power supply. As Scott Fisher said, there is nothing to induce any AC ripple or cause surges.
However - I don't know how long I would expect a car battery to last powering high amp chargers and stuff, and if you leave it in the car with the car running, you have re-introduced the problem of surges, etc.
As good as the newer power supplies are, (as promatchrcr said) and as reasonably as they are priced, and as easy as they are to carry (especially compared to a car battery) - I don't know why you would want to lug around a car battery instead of a power supply.
Theoretically - the car battery should be better.
Real World - power supply wins...