Looks like no one replied - I'm no expert here, but had the same question when my mom and I were choosing our itinerary.
From what I gathered in the Alaska Ports section posts and elsewhere is that the big ships rarely make it all the way to Sawyer Glacier in Tracy Arm - too much ice, etc.
This prompted us to choose the Hubbard Glacier itinerary (Northbound) for this September instead. We are trying for a small boat day excursion to Tracy Arm while docked in Juneau as apparently the smaller boats don't have the same issues getting to Sawyer Glacier - we shall see.