It is going to depend on which ship (Fantasy or Magic), which date, and also which port you want to start from (PC or Galveston). Your ports will be as follows for most of the itineraries
Eastern:
St. Maarten, St Thomas or
San Juan, St Thomas
Western:
Key West, Grand Cayman, Cozumel or
Grand Cayman, Cozumel, Costa Maya
We like St. Maarten a lot, there is some neat stuff to see on the island such as Maho beach, plus the people tend to be friendly and the island is easy to get around on. St. Thomas is okay but for us a bit harder to get around, the shopping areas seemed more congested and run down vs St. Maarten. We prefer the Western Caribbean ports. Key West is fun, the courtyard area always has interesting street performers, chickens run wild around the town, the area is very walkable and they have some interesting sites like a Ripley's museum. Grand Cayman was a beautiful island and we did a submarine tour here that we liked. We also found this place relatively easy to walk around. At Cozumel we went to an all inclusive resort for a cooking class and some beach time, we did walk around a bit to shop and the prices seemed to be good here. We have not been to San Juan or Costa Maya.
Another consideration is would you like more sea days or more ports. We find sea days more relaxing, so if it is ports we have been to several times we just take the extra sea day or at times we have just stayed on the ship and enjoyed some of the amenities to ourselves.
Discounts can vary, but I wouldn't count on them. The best value is going to be a cruise on the Magic and also to book the first available day.
DCL cruises tend to go up in price with time, even within minutes on the first day. If you are willing to wait there are sometimes last minute restricted fares that show up on cruise itineraries that are not selling out - but you will not be guaranteed a connecting room. The downside is a discount may never show up. So if you are set on an exact date I would not recommend waiting. They do run offers like kids sail free on the Mexican Riviera, but not always. Also during hurricane season and non holiday times the prices of cruises will be lower. Booking with a TA will usually get you at least some onboard credit, so that is some savings there. Booking through DCL will give you the most flexibility since you are able to change things your self by calling DCL or using the website versus having to call a TA. If you want connecting rooms in lower categories I highly suggest you book early or look for rooms that may be across from each other instead of connecting, such as one inside room and one oceanview room.