Briefly, as I'm on my iPhone--

The Major Marine and Kenai Fjords tours are very similar. The biggest difference is the meal on-board (KFT comes with an included chicken caesar wrap, whereas MM offers a full buffet for $19, which sounds good but can eat into wildlife-sightseeing time.

You cannot do a trip to Whittier on the ARR on the same day as a trip to Seward. It would need to be a different day, although at that point, it would be far easier/cheaper to drive back down to Whittier by car from Anchorage. It is feasible, though, to stop in Whittier when driving back from Seward by car, assuming you have time.