That comparison is a little extreme !
I would say pick the one that suits your times the best.

The ticket office for the goldliner is in the bus station. The aircoach service has a ticket office in the shopping area just in front of the bus station which might be easier to find than the bus. (Just walk out the front exit you will see it there - you cant miss it)

Why not spend a little time in Belfast while you are there? - the Crown bar is a real tourist trap .... i mean very popular with visitors ... and is just opposite the bus station. (Bus stations in Ireland are not the scary places they can be in the US.) Champ and sausages and a pint of Guiness would set you on your way to Dublin nicely !