I think the conventional wisdom is that it's cheaper to book the two legs of the trip separately. I've never done the straight "land and sea" trip, so I don't have any first hand knowledge. However, we once a did a sort of sea/land/sea trip, and booked all three legs separately. We did a 4-night Wonder trip, followed by two days at the parks, and then a 7-night Magic.
The thing I found about the land/sea packages are that certain resorts are tied to certain room categories on board. For example, we like to go with a verandah room on the ship, but don't need the corresponding "deluxe" resort on land. So we ended up booking things separately, rather than adding to two resort days as part of the 7-night cruise. That way we could stay at a moderate resort for the brief land portion of the trip, which ended up saving us a couple hundred dollars. This also allowed us to book the transportation we wanted, using the
DCL bus to go from the airport to the port, then renting a car for the trip from port to resort and back, and then again using the busses to get back to the airport at the end of the 7-night cruise.
In any case, it's easy enough to get quotes both ways, so you can compare and take what works best for you.