Virtually all the balconey cabins on Summit are identical. In addition, you will not want to sit on your balconey for the entire transit of the canal, you will miss way too much.
When going through the locks you will want to be up high and up front to best see what is going on. When cruising the canal there is much to see on both sides of the ship.
Once you are on the lake, perhaps some time to relax on the balconey is fine, but on the starboard side , you will not see ships crossing in the opposite direction as they pass on the port side.
If you can get an aft cabin for a canal transit on the Summit (or any of her sisters) DO IT!
Have had aft balconies on the Millennium and the Summit and they are fantastic without a doubt. The aft cabin balconies are much larger than those on the side and would afford a great view with the chance to have some shade from the covering.