I used Kaplan and UWorld.
I found both to be useful. There were actually similar questions on each that appearred on my exam.
The explanations on Kaplan were oftentimes subpar. But in actuality, you rarely need more of an explanation than the correct answer to realize why you were wrong on something anyways.
UWorld was a lot of fun. I used it to study for about 2 straight weeks. I started out getting anywhere from 40 to 60 percent correct depending on the type of material I was focusing on. But as I got deeper and deeper into it, my score would rise consistently. Their explanations were generally awesome, with great figures and animations. Very detailed about why things weren't correct as well.
After living on UW for two weeks, I dicked around with Kaplan for a week. Starting out around 50% and by the end of the week, getting 70-80% with consistancy.
I would say I did about 50% of the Kaplan Qbank as well as a decent amount of the Clinical Vignette thing. On UW, I went through the entire thing in a week, then just hammered home a lot of points until I could randomly pick any question set or any subset and nail 80%.
I felt okay after the exam, but I was shaky on some things. Some of that may have been holes in my own knowledge, but either way, the two banks reflect how you do against everyone else on their first try of the same questions. So you can get a good assessment of how you compare.
UW was a little more difficult than Kaplan. By that I mean a larger proportion of their questions are harder than the hard questions proportioned out in Kaplan. And their interface is better too, exactly like the real test. But overall, I think had I made it completely through Kaplan, I would say they're both equally good at prepping you, and a lot of synergism in tag teaming the two of them can only make you better.
Go with both. Do every question. Read every explanation. Then do them all again. Questions, questions, questions until you can't get anything wrong.
You'll know you're good when you can look at the answer choices, and know what the answer they want before you even read the stem. When you can do that, and if you do that on the actual test, you'll know you're $money$ 👍