There are two core differences between the LOMCE and the IBDP: the course structure and the type of learning encouraged.
The LOMCE will ask you to choose a study path and you will take subjects according to that path. For example, by choosing the Scientific, you will take Physics, Chemistry, and Biology instead of Geography or Philosophy. By taking the Social, you won’t be taking science courses and will instead focus on humanities. By taking the Artistic, you will do more theatre, music, and art than humanities or sciences. The LOMCE prepares you for depth of knowledge regarding an area instead of breadth of knowledge. In this aspect, it is similar to the A-Levels.
By contrast, the IBDP focuses on breadth rather than depth (although the course content is pretty detailed). In the IBDP, you will take two languages, a science class, a humanities class, a math class and a sixth class (which can be from the Arts [Group 6] or take another science or humanities course). This ensures you will build knowledge in various areas of knowledge.
LOMCE vs. IBDP
The LOMCE encourages memorization, evident in subjects like Spanish Language and Literature or History of Spain. As someone who took all four years of ESO (Obligatory Secondary Education), I would find myself spewing facts off the top of my head to answer questions like “When was X written” and “Explain the election results of X year.” LOMCE also doesn’t encourage much essay-writing, and when you are required to write an essay, it is fairly short.
The IBDP, on the other hand, encourages critical thinking. You answer questions such as “How important was the role of technology in WW1 to impact the outcome of the war” and “How is X social group portrayed in Y book?” There is a lot of essay-writing involved, which allows you to develop your skills as an effective writer. Each IBDP subject has an Internal Assessment of about 2000 words, whereas the LOMCE has no such thing.
The point where both LOMCE and the IBDP coincide is the large essay as a capstone project. The IBDP has the Extended Essay, a 4000-word research paper written individually, while the LOMCE has a Trabajo de Investigación, a 40-page research paper written in a group (maximum 3 people) which must be presented to a panel at the end of the second year (12th grade, or 2nd of Baccalaureate).
Both programs have their pros and cons, but I personally learned much more in the IBDP than the LOMCE. I feel that the IBDP goes the extra mile towards truly understanding what you are learning. If you prefer a more engaging, dynamic, and thorough program, take the IBDP and leave the LOMCE aside.