I love both ships; but given a choice for Alaska, I would prefer Island Princess, simply because the Horizon Court is forward instead of aft like on Diamond Princess. After you've been on deck for a while you can go inside, have something warm, and enjoy a spectacular panoramic view from the comfort of Horizon Court, a view you can't get on the other ship.
Ketchikan is good for just kickin' around; or the whale watching cruises are good there. In Juneau you should see the Mendenhall Glacier--by helicopter, kayak, Zodiak, or just a tour bus to the visitor's center. And in Skagway the White Pass Railroad is the "must do."