This is why it's always best to book with a reputable travel agent, preferably a Cruise Specialist, who can help you with all this and get you the best cabin for the best price.
With that said, I would go with the Verandah Deck as close to the middle as possible. I would not necessarily get a cabin on the Navigation Deck because it's right below the Lido Deck, which can often have alot of noise, especially if you're below the pool area or the buffet area.
If you're susceptible to motion sickness, you don't want to be in the front as that's where you'll feel the most motion. The back will sometimes have some vibration from the engines, and the middle will provide the smoothest ride.