My opinion only- very different. I would skimp elsewhere and do both, if this is not going to be a repeat trip?? How are you getting from Whittier??? There is supposed to be a cheap bus service available direct to Seward. If it arrives by 10- 10:30 am you could hook up with a Kenai Fjords boat trip leaving at 11:30. You could drops your bags directly at the depot for storage then walk to the Kenai Fjords office. Long day though.
If you can't do both, then my edge would be toward Kenai Fjords- for my wildlife interest. But I have done all of your list- certainly tough choices.