Both ships are very beautiful and well kept.
So, the major differences between the ships will be the itineraries.
The Star sails to Glacier Bay National Park, but skips sailing most of the Inside Passage. The Sun skips Glacier Bay National Park, sailing up Tracy Arm to see Sawyer Glacier, but does sail the entire Inside Passage.
The Star should visit Juneau, Skagway, Glacier Bay, Ketchikan, Alaska and Victoria, Canada.
The Sun should visit Ketchikan, Juneau, Sawyer Glacier, Skagway, Alaska and Prince Rupert, Canada.
The itneraries are half different, and where you rather want to go should determine which ship you should sail on.