Thing with BHD is that they are finding it hard enough to fit all current flights on the apron and expansion there wont be approved for a while yet. BFS is full (not to the same degree) as well but would be quicker to expand than it would be at BHD due to various obstacles.

BFS is out of belfast but is about as central as you could probably get in Northern Ireland to appeal to travellers from all over the provience as (as many belfast people forget) there are people outside of Greater Belfast!

A Rail link is probably something never to happen at Aldergrove as it would mean laying new track through a good number of miles through fields etc. The Airport bus is expensive (£9 return) but the Airporter is offering very cheap fares to BHD where a minimal bus fare to City Airport is available.