I walked both but I still prefer Fisterra. Maybe because it was the first. You never really forget the first one, eh
There's just a day or day and a half of different route to both (from Hospital) although for me the walk to Muxia can be more evenly distributed in 4 stages (20km+ something) if you stay overnight in Negreira, Sta.Marina, Dumbria and Muxia.
For Fisterra there are lots of possible stops and even nice overnight detours. I did it 4 times and rarely slept in the same village/albergue but the stages are shorter/longer than suggested above for Muxia.
Why I prefer Fisterra? First of all the stretch just after that factory (after Hospital and Fisterra/Muxia split) is the highlight for me. And first glimpse of the ocean nearing Cee. And descending to Cee on that knee busting incline with such a views I could cry. And then a stop in a bar in Sardineiro sitting almost on the deserted sandy beach (that's a ritual). And approaching Fisterra on its 2km long beach ...............
For me it's a beauty.
And I also prefer Fisterra over Muxia as a town too...
Happy planning