With a cruisetour, you need to Do your homework. There is SO much that is not understood with plenty of people making the wrong decisions.
This is in reference to the Interior Alaska portion. Go for as long as you can afford, the shorter cruisetours are all in transit, no time at destinations. NEVER consider any Natural History tour, always have the Tundra Wilderness tour included AND two nights at Denali Park. If including Fairbanks, ADD at least another full day, you see very little of this city on cruisetours. Best is to rent a car and get out on your own.
Seward is a gem. MOST cruisetours completely miss it. And some that "seem" to include it, the timeframe is way short of what you need. Again, do your homework and find out WHAT you wish to experience, see and do. THEN match it up with the right cruisetour. IT won't be easy.
Factor in ALL your transit times. Princess totes- their direct to Denali tour- but it is 10 hours on the train, with the best scenery for 2 hours Anchorage/Whittier. Rest can be so so, unless McKinley is viewable- rare.
Princess trains also are table seating, assigned seats, 2 riding backward the entire trip. The McKinley Lodge is NOT accessable to the National park, and requires a 2 hour round trip to the nearest town and rail station, Talkeetna. Plan your time wisely.
Budget 50% more for meals than you do at home.
Be aware, that independent touring options can offer you significantly more, if of interest. Can definately see twice as much for less cost. Very very simple to arrange in Alaska.