Have you looked at
Seat61?
In my experience*, there's not much to choose between the two ferry companies; maybe you could go out on one and back on the other. Both have bus connections in Dublin from just outside the terminal, at least for most sailings. The two terminals are almost adjacent.
On Stena you have to use a shuttle bus to get on and off the boat in Dublin, whereas on Irish Ferries there's a passenger gangway -- on one occasion I used Stena because of the slightly earlier arrival time, but this was was more than cancelled out by the wait until enough vehicles had disembarked for the bus to come on board the boat.
Taking some Euro coins for the bus into the city is probably a good idea (unless they take cards now). Seat61 says 'The bus accepts pounds as well as euros, but only in notes with change given in euros', but further down the page it says 'please have the exact fare'; my observation on one occasion (albeit some years ago now) was that sterling coins were accepted, but at a 1:1 exchange rate which made the fare somewhat higher.
Sometimes I think I've found that one company is easier to book SailRail tickets for than the other.
If you want to travel at a time other than the main departures at lunchtime from Holyhead or in the morning from Dublin, there seem to be some differences between Seat61 andd the current European Rail timetable with regard to which sailings take foot passengers; you may find that one company offers a sailing at the time you want and the other doesn't.
*A few trips over the past 15 years -- others may have more experience.
