Well, the bus is cheaper, and IIRC, travel time is about the same either way. The enterprise train service that does the Dublin/Belfast route is pretty nice. Seats on both the busses and the train are pretty comfortable. The bus leaves Bus Aras depot in Dublin; the train leaves from Connolly Station. I think both services end at the Europa in Belfast. It's been a while since I've done the Belfast/Dublin route, but I think the busses run more services between the two cities than the train.
Are your friends flying into Dublin and then travelling to Belfast? If so, the bus to Belfast stops fairly often at Dublin airport. If that's the case, it may save them a trip going into the city center to get the train.