It is my understanding that they will be very similar in terms of difficulty and question length, content, etc...
This is based on the fact that our director of clinical medicine stated that essentially the NBME folks write a number of questions for each topic (Ob, Gyn, Surgery, IM, etc...) and relatively randomly divide them into two groups, one for the shelf and one for step II. I have no other way of confirming this, but this is what she said.