Definitely take the Fishguard/Pembroke ferry and take the road from there.
Basically, the road from Fishguard/Pembroke is relatively high quality and turns into motorway reasonably quickly (as far as I remember). Good road whatever way you look at it.
The road from Holyhead down the coast is essentially a small rural road with all the same problems as you get here - getting stuck behind tractors, cows, steep mountain roads, etc . . .
I think the net distance (from Dublin) isn't much different whether you go via Holyhead or Pembroke/Fishguard, but the time savings and the ease of driving are significant.