Whittier is a great place to visit and (IMNSHO) worth a day trip....catch it on a good day and it is a spectacular spot.
If you want to see glaciers, don't expect to see them on the Valdez-Whittier ferry. Take a day excursion out of Valdez or Whittier specifically for the glacier sights.....in my view, you have more/better options out of Valdez, but that's largely because I like Valdez as well.
The Portage Glacier is a freshwater glacier that one used to be able to walk to from the road (long before the current B-B visitor center) but has now receded to the point of needing a boat to get to it.
As Joe mentioned, Whittier weather is often different from either the other side of the mountains (Portage) or the other side of the water (Valdez).
Personally, my favorite time to visit Whittier is in the late fall (early - mid October); admittedly, many of the services are shut down and it is unlikely to get sunny days, but the combination of saltwater, mountains, and in-your-face weather is a great way to remember that this is Alaska!
