USMLE requires more skill in clinical diagnoses. UWorld gives too many clues for each condition or pathology... i.e., for a heart murmur world is going to tell you exactly where it is loudest, on inspiration or expiration, exactly when in the cycle the murmur is heard, describe it to you along with any snaps or clicks, etc.
On Step 1, they might just tell you that a guy was playing ball and fainted, had a systolic murmur in the left chest area that is worse when he raises his legs when lying on his back, or something like that. i.e., you're expected to be able to work from a few less specific clues to a "most likely" diagnosis, instead of work from a host of very specific clues to a "has to be this, without a doubt" diagnosis that you see in world. This applies for everything... murmurs, back pains, coughs and dyspneas, headaches, etc. You need to know clinical medicine at a pretty thorough level (for a second year student) to quickly get to the "most likely" cause of the presentation described. It wasn't always clear cut.
World makes the diagnosis easy, and the second or third steps to answer the question very difficult. I felt like the USMLE made the diagnosis a bit trickier, but once I was confident of that getting to the correct answer was much more straight forward.
