I said this on another thread, but I thought the differences between the NBME assesment and UW were significant.
UW has much longer question stems, but is fairly easy to follow. You know exactly what the are asking even though you might not know the answer. They do have a fair share of zebras and nit-picky details in the answers.
The NBME test I took had shorter question stems, but were confusing as hell. I would just stare at the computer screen and shake my head... lots of distracting values and symptoms. Maybe it was just the difference in writing style that led to so much confusion.
I highly recommend doing atleast doing one of the three NBME tests before the real thing. The writing styles are drastically different and if one is too comfortable with UW, you will have a pleasant surprise with the real thing. Anyone else feel the same?