Barone,
All in all, I would probably say that they are both equally difficult (i.e., they are questions pulled from the same 'pool' with equal representations of all of the different topics). If you happen to find that one is harder than the other, it is most likely due to your knowledge base.
The GOOD thing about the NBME assessments is that they have an EQUAL representation of topics, whereas on the real thing, questions are pulled even MORE randomly, so you may get a block with 2+ very similar questions and/or many easy (or hard!) questions.
But I think for most people, through 350 questions, things tend to work out fairly well.
Best wishes!