Hubbard Glacier is spectacular. It is a very long frontage, and is indeed known for calving activity. Check out our Hubbard glacier pictures at
Cruising HAL Volendam to Alaska
The day we were there we got very close to the glacier. However there are many reports of ships being unable to get very close.
That said, consider that Glacier Bay is a World Heritage site and a National Monument. There is a reason for that. It is much more than the two Glaciers at the end. The long cruise through the bay is wonderful, and the ships will bring Park Rangers on to give you a complete commentary. The contrast between the two different types of glaciers at the end is amazing. And indeed Margerie Glacier is stunningly beautiful. That is why I, and many others, always recommend Glacier Bay for a first cruise to Alaska.
Check out our Glacier Bay trip reports at
Sun Princess Review
Cruising to Alaska on HAL Zuiderdam
Glacier Bay entrance is strictly controlled by permits issued by the National Park Service. That is why many cruise lines go to Hubbard instead of Glacier Bay. It is, frankly, second choice.
Hope this helps.