The caminos aren't too far apart distance-wise (e.g. San Sebastian is only around 80km from Pamplona) so in terms of temperature there's not too much difference, although as you know it can rain a lot more along the coast. I walked in October last year and there was a lot of rain, but we still had a great time.
I've done the Norte a few times and "road walking" isn't the first thing that springs to mind. I guess it depends on your expectations: some people expect a hiking trail, when actually it's a pilgrimage (i.e. comfort wasn't necessarily a priority when the local authorities planned the routes) and for a lot of the time you'll be walking along secondary roads. You'll also have to walk through quite a few urban and industrial areas, for example in and out of cities like Bilbao, Santander, and Gijon. However there are also gloriously rural bits in between, and in some sections you can take the GR-E9 path that runs close to the shore.
I think the biggest 'surprise' for those who have walked the Frances, is how few pilgrims there are. In October last year, we hardly saw another pilgrim during the day. The north coast is a popular destination in its own right, so the 'pilgrim experience' is very diluted; you certainly don't feel 'special' as a pilgrim. However if you just enjoy walking, and want to explore the wonderful culture and scenery of the north coast of Spain on foot, then you can't go wrong.