Belboy. When BD operated from both BFS and BHD into Heathrow, they publically stated that the demand and yield was much greater into BHD. Perhaps because they were a legacy carrier, more business focused, had more interlining traffic??; but that was the reason given when they then pulled out of BFS completely.
Its obvious that business people will pay for convenience, and business fares are higher yield, LCY fares being generally higher for example. And many business people flying to and from Belfast will find BHD more convenient. Now that was maybe about ten years ago, so have things maybe changed since then, I don't know, but would imagine business travellers from the mainland, if all things were equal, would prefer to fly into BHD, especially in their final destination was in the city.