I thought that the central route was good (i.e. Porto-Barcelos-Tui- Redondela - SdC), but I thought the coastal route was wonderful. I did the coastal route last July. There were not too many other peregrinos until the two routes merged at Redondela. I would not say that the coastal route was touristy, although it was built up in certain areas, especially at the southern end. It seemed to get a bit wilder and more natural the further north you go.
The difference with the coastal route is that there are a lot of different variants you can take. Some are marked and others not. For example, the Caminha da Costa is fully marked with yellow arrows from Porto to SdC, but it weaves inland for long stretches, so you are not always right on the coast. As I was walking last summer, I learned that there were stretches that were not marked, but ran right along the coastline. For example, when I crossed the river from Caminha into Spain, a few locals told me to follow the river out to the ocean and then follow the ocean north into A Guarda. It was a beautiful day, there was a good trail, and it was one of my favourite sections of the entire walk. If I had followed the yellow arrows, the route would have gone inland over a hill (which may have been nice as well).
Another example was on the way into Baiona. The route was following a coastal road, but there was a wide pedestrian/bicycle lane so that it was not dangerous. The road ran right along the coastline. If I had followed the yellow arrows, they would have taken me inland at one point, up over a few hills (which I heard later from two peregrinas was quite tiring). I stayed on the road, right beside the ocean, and stopped for lunch at a hotel up on the hill overlooking the ocean. It was marvelous. I then just followed the road into Baiona.
So I guess what I am saying is that I have a slight preference for the coastal route (up through Vigo), but the route and options are a little less certain.
From my personal experience, the central route was somewhat busier (although not too busy). However, I remember someone posting here not that long ago that there were reasonable numbers on the coastal route. I saw maybe 4-6 people per day on the coastal route last year, until the numbers increased significantly at Redondela.
Either way, you would enjoy it.