Flew from Denver to Frankfurt via Lufthansa - roomier seats, better food, predinner drink/ 2 glasses wine with dinner/ after dinner drink for only fifty bucks more, plus hot towels to freshen up. And your own TV, choice of a dozen movies/ dozen channels/ live gps.
However: allow three hours to change planes in the Frankfurt airport, since the plane will land somewhere in the outlying regions of the airport, then you go through customs a couple of times (once while transferring concourses. More if you don't know about the tunnel between concourses b & a. If you have extra time, try Herman's Sausages, a cart somewhere around A28.
Stayed overnight at the Best Western near the airport ($118 on Expedia). OK motel to sleep in, clean. 4 star restaurant (Belvedere) upstairs - great Italian food for dinner, good breakfast included. 10 dollar taxi from the airport.
Typical RCL cabin. Go the twin route, that way you can share the community space in the center of the cabin.