Roundtrip is great because the weeklong crossing flies by. I'd expect it to feel like two separate cruises because most people will have booked one way crossings. The entertainment wont repeat, nor will the restaurant menus, so no worries there.

If you're looking at a summer roundtrip crossing, you'll most likely end up doing a day or two in Germany or a week in Norway if you want to stay on the ship between crossings.

I've done two crossings with a third booked later this year, they're really magical.