I only used USMLE World, and I thought it was a fairly good predictor of the difficulty of the questions asked. The other thing that I liked about it was that the interface was exactly the same as the real USMLE exam.
I didn't do well on USMLE World, to be honest, and it did scare me. I feel that, in retrospect, I should have done all the questions (and started doing them sooner); I only finished about 50% of the total questions by the time I took my exam.
I guess it doesn't matter anymore, though :-)