question is how long are you going for?
short trip use the airport parking, like others have said North terminal parking is better then south, beware they are updating the N terminal looks, but you can walk directly from the parking to the airport,
Long trip, use off site parking, and yes the shuttles do take time, and upon arrival the walk to the shuttle area can be quite a hall,
also keep in mind, due to the construction in the North terminal, the shuttle buses now are only dropping passengers off at the South terminal.
hope that helps