I just took USMLE I.....
I used both USMLE World and Kaplan QBank... and I have to to say that they are both QUITE different from each other...
As far as which one is more similar to the actual exam, i would have to say Qbank. DEFINITELY. I'm not sure how people came up with the idea that UR is like the actual thing.
HOWEVER!!!! UR is a GREAT RESOURCE.... The questions are very difficult and up to 3rd or 4th order thinking... (The actual exam is not---> perhaps 3rd order at the most) This makes you really think, and the explanations are SUPERB, much better than Qbank, and you will learn alot this way....
Personally, what I would do is as many UR Q's at the begining of my studying because the explanations are so thorough and will supplement your learning....
THEN, I would use QBANK on RANDOM MODE!!! If you do QBank on Random mode, you will practically be taking the actual exam, IN MY OPINION.... Of course, when you select random mode, you wont get the actually porportion of questions from the different subjects.
USMLEWORLD: More concept based and less fact based questions.
Kaplan QBanks: More fact based questions and concept based questions.
Either way, get your studying done early and do many many questions... During the exam, there were some questions that I only KNEW and REMEMBER because of the questions I did.