Welcome to cruise-chat! We recently took our first Alaskan Cruise and did the helicopter glacier/dogsled tour in Juneau. It was through ERA helicopters. They flew us over various glaciers and then we landed on a dogsled camp where we got to ride the dogsled for about 45 minutes (my husband actually got to be one of the mushers on our team). It is an experience that I would recommend to anyone, it was an amazing tour. If you want more information just let me know. I know you can have the same type of experience in Skagway, but that was not one of our stops, so I don't have any firsthand knowledge of the Skagway tours.
What cruiseline are you going on? We went on Princess and booked this excursion through Princess.
Here is a link to my online photo album so you can see what Alaska was like for us. It is an absolutely breathtaking experience. http://travel.webshots.com/album/559114699QRoTvk