Too add to this........ Prestwick is the only airport in the central belt that has its own railway station........ Working in the railway industry as I now do, I know that passengers like to get off a plane. quickly get on a train with minimal fuss and get where they are going without having to get on buses, go round housing estates etc etc........
It looks like it will be a good few years before Glasgow & Edinburgh airports get their own railway stations.........
Personally, I prefer flying out of PIK as its a lot more relaxed than GLA & EDI......... and its easier to get to !
Ian H