I am currently on a Hurtigruten cruise, Day 10. I started in Bergen, went up to Kirkenes, and now we are heading back to Bergen, where we will arrive on Day 12.
The scenery has been good throughout, and it changes, so it's not the same everywhere. However, I would say the section between Tromsø and Trondheim was overall better than the secction between Bergen and Trondheim.
The excursions are different northbound and southbound, because of the time of day arriving in various ports. Since you are only going southbound, you won't have the same excursions (Geirgangerfjord and the Trollstigen road) that I had going northbound. And I don't believe the ship will be entering Geirangerfjord at all on the southbound route.
Tomorrow night I will be doing the excursion on the Atlantic Road, which is supposed to be pretty spectacular. That is south of Trondheim. That's the only thing you would miss if you go by another means of transportation. There is also the option of a very challenging hike in that part of the cruise, going from Kristiansund to Molde.
If you PM me, I can give you suggestions for excursions on the southbound route from Tromsø to Trondheim, both ones I did that I enjoyed and ones I did that I don't recommend.