you cannot compare Princess and Costa.
I would prefer Princess and Norway and all the Fjords are so beautiful. Something like Alaska, I wish we could book it, but itīs really difficult for non US-citizens to book a Princess cruise.
The Costa Magica is more Carnival, more colours, more italian people, different kind of passengers, not the kind of co-passengers I would prefer and not the kind of design I would prefer. I only can write you what Iīve heard from other passengers and their experiences.
Sea could be rough around Scandinavia and the North Cape but it also can be very smooth, but it would be a cruise you will never forget.
Friends just came back from their cruise on the Constellation and they had wonderful weather, no waves, only a little rain, absolutely perfect.
But you have to make the decision, we can only give you some advices.