The problem with bus links that don't have segregated running is that they are at the mercy of prevailing traffic conditions. All it takes is one broken down lorry in the wrong place and people miss their flights.The bus link (500) is fantastic. Spend a wee bit of money on that to improve further rather than tens of millions of a rail link that will be poorly used.
Yes, trains can break down too but it is much less common an occurrence and, generally speaking, trains deliver a reliable service.