I haven't been through Tracy Arm but would say the Hubbard Glacier is best. A friend of mine booked on the Regal last summer and due to bad weather, had to detour and missed Tracy Arm. We went on Infinity in May 2005 and the Hubbard Glacier is where we took most of our pictures! It is an awesome sight! Dress warm because it gets chilly up on deck. We had a family veranda suite and so stayed on our veranda for the duration -- less windy and much warmer, plus we could retreat to the inside for some hot cocoa (courtesy of room service, order early!) when it got too cold. The Hubbard calved about once every 3-5 minutes and each time was spectacular! Watching cruise cams for about a month afterward, I never saw a Celebrity ship rerouted from the Hubbard for bad weather. If going to Sitka, I highly recommend the Sea Otter & Wildlife excursion. Though pricey, it's well worth it! We saw humpback whales, seals, sea lions, sea stars, puffins, eagles, otters, and much more!
Have a great time!