1) I have only booked a balcony room once - because we booked "so late" that there were no Cat 9 rooms available (early Fantasy sailing). Been upgraded to a veranda room twice - and we really never ever used the balcony - it was either way too hot or too windy (once too cold). The obvious advantage of a balcony on Transit Day is that you don't have to "fight for space" up on deck 10. You can sit and watch the canal slide by. But you have a limited view - you're not seeing what's happening on the other side of the ship, nor what it is like sailing into a lock from the front of the ship or out from the back of the ship.
2) No I don't think so. But the real estate trueism holds here for all sailings ... Location, Location, Location. The higher the deck the higher the price.
3) Since our 3rd cruise (we've now been on over 20
DCL cruises) we always book Oceanview rooms on Deck 2. For us it's a great value. And LOTS of stairs (get to eat more desserts that way). And there are plenty of public areas that are outside (deck 4 IS the Largest verandah on the ship!!), decks 9 & 10. Promenade lounge has seating by the portholes as does the Cadillac lounge in the After Hours area.
Just about the Canal Transit...
There is a LOT to see on the Canal Transit day! I don't know what it is like with the new locks that the Wonder now uses, but "back in the day" we would be up on deck 10 - to get the overall view of the locks, the process, the "event".
Being on deck 10 gives you easy access to either side of the ship. It's tough to get a spot all the way forward - LOTS of people trying to do this. But throughout the day there will be opportunities as people come and go. The views up here are great. An interesting thing is that you really don't feel yourself (ship) moving up and down as you go through the locks. But take a look at the light posts along the canal and you can tell that you are rising up or sinking down.
But deck 10 is NOT the be all, end all. Our first transit of the canal we spent the entire day (14 hours) up there. (Note: this was only the 2nd transit of the canal by DCL and they do it a lot quicker now -- again old locks). And as it turns out we missed some other interesting views. Deck 4 is an amazing view of the proceedings. As ships are raising or lowering, you can actually tell its happening. You get a great view of the workers and the workings of the locks. And you get a bit of the scale of the container ships. If you are on deck 4 watching, I would station myself by the doors so you can easily get to the other side of the ship to see what's happening over there. And since it frequently downpours in the canal zone, well you are under cover on deck 4.
Now for the deck 2 perspective ... again, old locks ... you most definitively would NOT want to spend the entire transit in your room looking out of the porthole. BUT you do want to be in the room as the ship enters or exits one of the locks. There is less than a foot clearance on each side of the ship. As you are sailing towards the lock, you have the view of the canal's "river". If your room is on the correct side, you will then see the "dock" that extends out from the locks. You are a little ways above the dock. Then as the ship enters the lock, you see the "ground" (concrete) start going uphill and and then all you can see is the wall, then you pass the door to the lock and actually enter the lock. After the ship is situated, they start filling the lock with water and you see the concrete wall move - your eyes are telling you that you are rising! then you can see the top of the lock, and then eventually you see the lock / dock below you.
Back on Deck 10 I would want to be up there to see your passage under the Centennial Bridge and the Bridge of the Americas (both on the Pacific side). I think that they are working on a bridge over the canal on the Caribbean side, so I would be up on deck 10 for that as well. There's something magical sailing under a bridge.
4) RE: Motion Sickness and feeling the movement of the ship: actually being on Deck 2 is much better than being up higher - and mid ship. Take a pencil and hold it between your thumb and finger at one end. Now rock it back and forth -- where does it move the most? At the top away from your fingers or at the bottom where you are holding it. In this example, the pencil is the ship and your fingers the ocean. The farther away (UP) from the surface of the ocean the greater the motion.
I have only been impacted by motion sickness on the ship about 3 days (out of about 150 days at sea), but one of those days was on the way to Cartagena. And generally speaking, the Pacific is much rougher with more motion than the Caribbean or Atlantic.
Hope this helps. Have a great cruise!