I know this isn't what you're asking so this is completely unsolicited, but we wanted to do both Mendenhall and whale watching and I exhaustively researched all the options (including the one you are asking about). So, I will tell you that Mendenhall can get busy, especially with tour groups. We ended up doing both on our own. For Mendenhall visit we rented a car at the dock from Juneau Car Rental Company for $60 and drove to Mendenhall first thing. We arrived at 8:30 AM and pretty much had the whole place to ourselves and it was GLORIOUS. There are no other people in our photos, we had all the views without waiting. It was quiet and beautiful.
Then in the afternoon we returned the car and did private whale watching.
So keep this in mind as an option depending on what you find out about this particular tour.

I hope this helps you weigh the options!