I have not been on the Norwegian Spirit (but will be on it 09/05) but I have been on the Star. (06/04). I was, however, on the NCL Sky which had the same itinerary as the Spirit, with the exception of Prince Rupert vs Victoria.
The Spirit goes into Tracy Arm and back to Sawyer Glacier which is a beautiful thing to see. Tracy Arm and that journey in and out of there is really, very beautiful. However, the whole thing only lasts a couple of hours and your time of actually viewing the glacier is minutes.
On the Star you will go to Glacier Bay, which is from early morning until mid-afternoon and you see many glaciers, as well as wild life galore. Going into Glacier Bay was a truly magnificent experience and I would love to do that again.
One thing changed from when I cruised to AK in 2003 vs 2004. In 2003, in Juneau we docked right in town. You literally walked off the ship and right to the shops. Now NCL has a dock a little bit further down and when you come off the ship you have to walk to a shuttle bus and ride into town.
All in all, you can't go wrong with either cruise!
Hikini