From what I've seen around these threads, no one can predict whether you'll find the Step easier or more difficult than the NBME. Just for clarification, are you talking about the NBME free practice set, the NBME assessment exams (that cost like 45 bucks each), or the NBME comprehensive basic science exam?
I've taken both the comp exam and the free exam, and felt both were pretty easy. The problem is, people always talk about how the actual Step 1 was more difficult than they had expected, so I would certainly expect it to be more difficult, but hope it's easier. I think the worst thing you could possibly do is underestimate the real thing because you're killing the practice tests.