Yes, you can choose which Deck you would like to be on, sometimes it's a matter of choosing the category you would like. For instance, the cabins on Deck 8 are in category 4 and above, and they are the most expensive.
It's a matter of your personal choice and / or your budget balanced within your choice.
As for the Itinerary, it's also a matter of choice. I really enjoyed our cruise in January to the Eastern Caribbean, versus my DW is looking forward to a cruise on the Western route and seeing Mexico.
All of the months seem to have a lot of people on the
Disney Cruise Line. It's very popular because people enjoy Disney nad they only have the two ships.
If you want a cabin with a veranda, it will have to be on decks 5 thru 8. Otherwise, decks 1 and 2 are also available. If you want a cabin with an Oceanview Porthole and not a veranda, it would be Category 8 or 9, which is on several decks.
So, my advice about choosing for yourself:
Decide which Itinerary is more interesting to you.
Decide what month and week fits into your personal schedule.
Decide what category of cabin you like and fits your budget.
hope that helps