Gillyweb, my wife and I walked from Porto in April this year and loved it. We walked via the central (what some call the interior route). The only thing I would suggest is that in hindsight we wish we had walked day 1 on the coastal route due to a lot of the first day walking along the narrow shoulders of the busy N-306 highway. Also a lot of walking on cobble stones and the granite setts which are hard on the feet. Once you reach Vila do Conde you can branch back to the central route and rejoin the central route at Arcos. We enjoyed all of our days in Portugal, except as mentioned our first day out of Porto.
As far as safety, I think their is no difference between Portugal and Spain. As you know, you can always walk with others or at least keep other walkers in sight to improve your feeling of security. The Portuguese people are lovely and helpful and seem genuinely happy to see you.
Overall we found the trails very well marked and literally did not put a foot wrong our entire time in Portugal. We will certainly be back at some point for another Camino in Portugal.
Staying with the familiar (
Camino Frances) has it's attractions, but as they say, variety is the spice of life!