I think the costs are equivalent, but Sky is a far superior service imo. The big advantage Sky has is the interactive red button. While it's not great during a game (different camera angle, player camera, etc), it's excellent on a Saturday evening. You typically have your choice of 6/7 Premier League games that you can watch the extended highlights from. If your son is a supporter of a particular club, it means that the sometimes short coverage offered by RTE2 or BBC1 on their highlights programmes can be avoided by using sky.
Also, on Champions League nights they have 6 or 8 games to choose from, again by using the Red button.