The big ships (and even the Mariner counts as one for the purposes of this) have a hard time getting all the way into Tracy Arm. So, yes, the Mariner does do a Tracy Arm sailing but you'll be more up close and see more things on the excursion than if you stay on the Mariner. The smaller ships can get much closer to the glaciers than the Mariner will so you may have a chance to see calving up close and personal (though hopefully not TOO close and personal) as well as more wildlife. And not all cruise ships get to do this excursion as well. There is no similar excursion at Hubbard Glacier or in College Fjords on the other cruise lines. Only the Mariner and the Norwegian Sun offer this tour so it's somewhat unique.

People that have done this excursion have regularly rated this excursion as their most favorite excursion that they did in Alaska.

http://www.allenmarinetours.com/sightseeing/juneau/tours/glacier-fjord/index.html
(this is the description of the tour though we'll be boarding the catamaran directly from the Mariner rather than in Juneau and getting dropped off in Juneau rather than back at the ship)

Masaki