I really don't know about this actually, and I've wondered the same thing and done searches for it on here and stuff. The consensus on sdn seems to be UWorld twice, but I just find it kind of hard to believe. I mean you'll remember the answers to at least some of the questions you've done before so you lose that aspect of missing a question and then being like **** better study that. Also I kind of like USMLERx b/c it is kind of like going through FA again but with questions to quiz yourself. Its definitely easier though and the explanations are shorter. That makes it easier to get through though. Who really knows. A lot of people have asked this question and I think its a matter of opinion, but more people seem to have the opinion that World x2 is better for whatever that is worth.