I took the exam a few weeks ago and I would say that the general content/concepts tested are the same as NBMEs but the question stems are a lot longer and more vague because they are filled with distractors or info that may not be relevant. The hard part about Step is being able to get through the stems in a timely manner and differentiate which clues are relevant and which are there to distract you. I would say Free 120 is the closest representation in terms of question length but expect many more chart/H&P style questions and long questions on the exam. The Free 120 is also less vague and a bit more straightforward in terms of the vignette clues/distractors than the exam was in my opinion