Transiting the canal from West-to-East (which is actually more South-to-North) is the ideal passage through the Panama Canal. Since the canal is more south-north oriented and you enter from the Pacific side at about 8 AM in the morning (Panama Time) the sun will be coming from the Starboard, rear quarter of the ship.
If you book a cabin on the Port side (left side, facing forward) you will look out of your cabin to the west. Ususally,the canal ship traffic stays to the right (as we drive on the right side of the road) so on-coming ships will pass to your side of the ship. With the sun coming from the other side of the ship (Starboard) and you facing West, what you see is in bright sunlight and great for photography.If you were to travel from the Caribbean to the Pacific, you would be traveling south, facing into the sun and looking at objects in shadows (not so good for photography). So book a Port side cabin!
If you will be taking a load of photos as many people do, the sun will be behind you and illuminating the left side (West side of the canal for some great photo opportunities. Of course the sun will reach the port side of the ship after noon, but some of the best photos opportunities will occur before noon. In any case, you just might want to find a place at the bow of the ship and at a high vantage point and watch both sides of the ship as you transit the canal.
To get you prepared for your canal passage, here are a couple of web sites at the Panama Canal to give you some background history and operating information on the canal. The first one is the homepage for the Pamana Canal Authority with the background information. The second one is the 24 Hr. stop-action Canal-Cam that you can watch at your computer day or night. For the best thrill of all, in late Sept., the cruise ships will start transiting to the Caribbean Sea and for six to eight weeks you will have the opportunity to watch the cruise ships pass through the canal. Cruise ships pass through the canal only in the day time.
The Canal-Cam is located at the Miraflores Locks: the first set of locks on the Pacific side. The canal is on Easstern Standard Time, but they don't participate in Daylight Savings Time. So, 8 o'clock Central Standard Time is about when the cruise ships will enter Miraflores Locks. Passage through the canal takes about 8 hours. Enjoy your canal passage planning.