EverQuest Next will be subdivided into two segments: a standard, quest-based MMORPG, and a world-building tool called EverQuest Next Landmark.
Applications for Landmark beta-testing are already being accepted, and John Smedley has said that the best of Landmark's player-made worlds will be brought into the MMORPG.
So landmark is the world building tool, and the MMO that will be derived from their landmark exploits will be EQNext.
There is nothing that i can find that says if you opt into the Landmark beta, that you will get into the EQNext beta.