I think a dose of reality is required here, instead of the usual Edinburgh versus Glasgow slanging match. Both are cities that require an airport, with a wide(ish) range of destinations.
Edinburgh has had phenomenal growth over the last few years, and because it has the stronger inbound market, it will continue to gain routes, airlines and passengers from continental Europe and possibly North America along with developing a smaller niche in the bucket and spade routes.
Glasgow, will probably stagnate a little in terms of passenger numbers airlines and routes, with routes continuing to be dominated by the bucket and spade destinations and domestic flights.
Prestwick, is harder to call. It will depend on what Infratil decide to do.
I think it will either continue with Ryanair at around the 1.3Million pax per year and sunshine routes or become an engineering hub, with limited cargo and passenger flights.
I think that the development of EDI or GLA as a hub is complete pie in the sky. Glasgow is too constrained by the M8 and the Black Cart, while Edinburgh does have space to the north(ish) of the runway, but under the current environmental regulations, consent would be difficult to obtain.
Thus, we're going to continue with the two major airports in the central belt and the eternal slanging match