Our family of four (kids age 10 and 15) just finished the Tracy Arm Inside Passage itinerary on the Grand Princess. We booked two cabins, a balcony and an inside cabin across from it on the Caribe deck. A balcony cabin is an absolute must when going to Alaska. Both cabins were decent and the beds are very comfortable. The inside cabin seemed to be showing less wear than the balcony cabin - I'm sure those are the most popular. We saw seals, whales, and bald eagles from our balcony cabin that we might have missed otherwise.
The ship does have some wear but overall was nice. The buffet food was decent and there is always plenty of it, although very crowded at times. If you go during off times, you can find a nice spot at the window and get great scenery on the buffet/deck 14. The food in the main dining room was decent as well, and a little less chaotic than the buffet. If you want to spend a little extra at The Crowne Grill and Sabatini's you will get a noticeable improvement in food quality and atmosphere.
We weren't really interested in the entertainment much, but we did like the magician who has two different shows and we saw both of those - he was very good. The kids like the golf and ping pong tournaments and the MUTS are a lot of fun if not too cold and windy. There is an indoor pool (Disney does not have an indoor pool on the Wonder) which is very nice and bigger than most. The ship's naturalist (Mark Harris) narrates through several key points of interest (Tracy Arm, etc.) and we really enjoyed listening to his commentary.
Alaska: wow - incredible. Do NOT miss one second cruising through Tracy Arm fjords, it is simply amazing. Do as many shore excursions as possible, that is the whole point of an Alaskan cruise, afterall. We did book ours through Princess and they were all excellent. In Ketchikan, we choose a lumberjack show (somewhat cheesy but for the kids mostly), Mendenhall Glacier and Skagway White Pass railway. The bus driver that took us to Mendenhall Glacier was fantastic. It was like listening to Rick Steves narrate your drive through Juneau. The White Pass Railway in Skagway has incredible views and we saw a bear and caribou during the ride. In Victoria we just took a cab to Butchart Gardens which worked just fine and the gardens are like nothing I've ever seen before.
We debated the Princess Cruise vs. Disney and settled on Princess because we could get a balcony at a reasonable price, Princess offers anytime dining (vs. set dining times) and Princess leaves from Seattle instead of Vancouver. We were not disappointed.