Irene,
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 it´s 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 I´ve 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.