Like most people, I did Forms 2 and 4. We also took the comprehensive NBME shelf exam at the end of the year through our school. I got a 240 on the comprehensive shelf, then 2 weeks after studying, got a 235 on Form 4, and two weeks after that, got a 230 on Form 2. Needless to say, I was a little worried about my actual score since my scores kept dropping, but I got a 249 on the real thing. So, from my limited experience, maybe form 2 is harder?
In my opinion, the NMBE practice tests give you a ballpark estimate of where you will be, but not to the point where they are accurate within a couple of points. Last year on Step 1, I did worse on the real thing than the practice NBME test. (245 on the practice, 238 on my actual Step 1).