Basically down to personal preference, the Irish Ferries sailing leaves earlier and arrives later than the Stena service, The Irish Ferries service can be delayed sometimes due to freight volumes (load on load off) and also the level of traffic in Milford Haven. Stena normally runs to time, but this can obviously change on the day.
Coming from Wales you will be on the road first in Ireland with Stena which depending on your destination may make no difference or be important, going the other way (back to Wales) you normally clear the Pembroke exits before the Irish Ferries traffic unless there are road works or the usual slow moving driver ahead, again this may or may not be of use.
Why not go on both, outbound on one and return the other, experience both methods of travel the modern Isle of Inishmore and the revamped Stena Europe and if you can stretch to it upgrade to Club Class and Stena+ to see the best that both ships offer.
If you are a ferry fan, contact either operator in advance and you may be able to get a bridge visit organised, well worth the effort as you will get to see how the ships work.
The Inishmore is a very fine ship and offers a very comfortable crossing if sometimes lacking a little bit in general upkeep, for some reason Irish Ferries don't seem to invest in white paint to cover the rust as much as they should, the Stena Europe has recovered from it's intial introduction replacing the Koningin Beatrix on a route that with the Stena Felicity before the KB that was quite used to having some amazingly well appointed ships with lots of features never before seen on the Irish Sea as anyone who visited the Felicity's cinema would agree, however Stena have invested quite a lot of Euro's in the Europe and it really is a fine and comfortable ship to travel on.
Either way, it is a lovely crossing and much better than flying.