I use Stena every time. The interior of the boat is much nicer. The children love the softplay and movies. As we travel 4+ times a year we get free lounge access. Irish ferries lounge is a complete rip off and waste of money, Stena's lounge is an oasis of calm.(Unlike the softplay area!)
The most important reason I pick Stena instead of Irish Ferries is down to Irish Ferries treatment of it's staff in 2005. They sacked all their Irish and UK staff and replaced them with cheaper eastern european staff. Irish Ferries staff seem very unfriendly and surly, Stena Staff seem better turned out and offer better customer service. The roads from Pembroke and Fishguard to the dual carraigeway at St Clears, Carmartheshire are equally bad! Pembroke is a little closer.