I sailed aboard the Constellation which is virtually the same ship. I can't say anything about the balcony cabin.
You sailed the Mercury, so you know the types of activities and style that Celebrity is. Summit will offer the specialty restaurant, a choice of spa food for lunch, and pretty much the same buffet and dining menu in the main restaurant. Summit has also been fitted for Cirque de Soleil, "the bar at the edge of the world", and there's one masquerade ball planned where you can make or buy a mask. I think the specialty restaurant is wonderful, and you should dine there at least one evening.
There's no equivalent for Mercury's Pavillion lounge, some of the activities that take place there are moved to the observation lounge, but there's less activities on the Millennium class ships. The showroom entertainment will depend on the quality of performers that are booked at the time of your sailing, so they may or may not be better than what was on the Mercury. For the most part, Celebrity isn't known for the quality of their showroom entertainment.
My opinion is for Alaska don't stay in your cabin to view the scenery. If you are going in the inside passage scenery is on both sides of the ship, so I think it is best to go out onto deck space in the back or front of the ship. Like the Mercury, there are large windows in the observation lounge to view Alaska, I don't know how much of it is portioned off for Cirque de Soleil, however, and the windows aren't quite as large as those on Mercury's.
Alaska is wonderful, dress in layers, it has become more crowded and touristy with more and more and larger cruise ships visiting the Alaskan ports.