I have done both southern Norway and all the way to Svalbard on the same cruise. We were already at 80 degrees north headed up to the packice when we had a medical emergency and had to return south.
The southern part is much prettier with wonderful fjords. Lofoten Islands aren't that spectacular. North Cape is neat to see but not that spectacular (went to ice bar). Longyearben is interesting but not much to do. Scenery of Svalbard fantastic. Sailing through ice flows was wonderful.
If you have never been to Norway, try the sourthern trip. If you don't mind sea days and our interested in seeing some fantastic scenery (and maybe a polar bear), then head up north. If you really want to see Svalbard and maybe Bear Island, try an expedition ship such as Silversea Silver Explorer instead of Celebrity.
No matter what, you won't make a bad decision as Norway, all of it, is beautiful.