On the 5/11 Infinity we stopped in Juneau, Skagway and Ketchikan plus Hubbard Glacier.
Needless to say it was a great cruise but I could have skipped Ketchikan. Since the average rainfall is 161 inches a year, it was cold and rainy all day. In addition, Holland America and Princess were in port at the same time so we had to be tendered to shore from the Infinity. All the shore excursions were full so we ended going back to the ship early. Skagway was my favorite...nice small town, easy walking distance to all the shops. We booked the White Pass Yukon railway trip (3 hours) and were not disappointed. It's not to be missed. In Juneau we booked the helicopter to the top of Mendenhall ($297 each)...what a disappointment. The skies were overcasted and we could barely see a 1/4 mile in any direction.
I've heard Sitka is really nice too however I've never been there. You will enjoy Alaska. We are looking forward to the Southern Caribbean on the Infinity next year.
The main consideration you need is 4-5 days added interior touring on any one way trips. I greatly prefer southbound trips for the last sailing day loaded with marine wildlife. Sitka is a prefered port- you have some tough choices.
When you make your helicopter arrangements, don't do it through the ship. You can do it on your own through Coastal for about $160. And there is a great wildlife tour in Ketchikan. You can fly to an area and see bears catching salmon as they try to jump up a waterfall. We went last year (round trip cruise on Galaxy) and booked all our tours on our own and saved a lot of money. This year we're doing the southbound on Mercury and spending an extra 4 days in Denali. We really feel we missed a major part of Alaska by just doing the cruise.