If you only use one question bank or the other you'll love either kaplan or UWorld.
If you use kaplan then use UWorld and try to go back to Kaplan... doing Kaplan questions/explanations will make you want to punch your screen.
That being said, Kaplan has great "What would Jesus Do?" questions and many good biostats questions not covered by UWorld. I would recommend splitting a subscription with a few friends for a month and going through all of those questions.
A poll was done of the 3rd years at my school. Here are some quotes from the survey open response section:
"know that the Kaplan QBank questions are definitely harder than the real deal."
"usmle world questions were more like the real exam"
"If I had to do step 1 all over again (thank God I don't), I would just get USMLE world. You also don't need Kaplan for 9 months. Most of you won't use it for that long. Most of you don't need it for more than 1-2 mo."
"I didn't use USMLE World, but I heard from other students who did that it was much better than Kaplan QBank. If I were to take Step 1 again, I would use USMLE World."
"QBank is good for going over content, but the scores on QBank don't correlate well with the actual test. Instead, the free sample tests available from USMLE are the most representative."
"Kaplan is the only dealer (to my knowledge) that offers a full length (ie 7 block) mock exam. This is sort of silly in that you can generate seven 48 question blocks yourself in any question bank program. But it is nice to be forced to do all 7 in one day and to not have any feedback between blocks regarding your progress. They also give you a feedback break down at the end. As far as I know you can only acquire this practice exam by purchasing a Kaplan Q bank."
Hope that helps. I have Qbank, but I have been finding that the above statements are true; Qbank is overkill as it's WAY harder than most of the questions on the real exam. that being said, it's better to be over-preparred, right? I bought Qbank+ mainly because I wanted access to the stats.
I don't think qbank questions are necessarily any "harder". I thought I was getting similar percentages with both question banks at that point in my studying. To me the difference was that with Kaplan i had more "that's stupid I'll never need to know that" moments (especially microbes) whereas with USMLEWorld they were more often "damn I need to understand that concept" moments. I thought World was probably "harder" and a lot of people were beating themselves up over lack of improvement in their world scores.