They're a little bit different. If you go back to really old tests (before that), you realize that Step 1 used to be way different. There were virtually no clinical scenarios. There were just straightforward questions like "which of the following proteins is mutated in this cancer, etc.". They've been trying to make the exam more clinically oriented with case vignettes. Regardless of the new format though, the exam tests the same facts and concepts just in a different manner.
I've taken 5/6 and they're not too far off from the new ones. If you only had money for a certain number of them, I'd say 11,12,13 and 7 are the best ones that are more representative of the newer question format. At the core though, this is a knowledge exam. So, if you're OK with the clinical vignette format (and the knowledge core is just what you really need to test), I don't think the older exams are necessarily bad predictors of how your studying is going. They just might be a little farther away from the format the test takers have been moving towards.