How long before you travel? There are often good deals if you book mid term car park a few months in advance. It is a smaller car park than the long term though, so sometimes finding a space can be tricky unless arriving in the early hours. If walking is an issue do the meet and greet, as you can't rely on parking close to a bus stop in the car parks.
Long term car park is a last resort for us now, it takes ages waiting for one to arrive, and then get to terminal building/back to car park.