The upperclassmen at my school have unanimously recommended UWorld.
I had a kaplan trial and have used both.
Head to head, Kaplan has better aesthetics (the FA page references are nice) but I get the feeling they might be focusing on some obscure details on occasion, even if they can find a FA reference for it. They also have AV questions which UWorld didn't have when I started my Kaplan trial.
As mentioned above UWorld's explanations just feel and sound like I'm getting more out of it. I feel like I'm getting a better experience with UWorld.
With Kaplan's questions, I have a hard time gaguing where I should be... Did I get it wrong b/c it was an important concept I should be studying or did I get it wrong b/c it was something someone thinks is important and I'm going to waste my time if I focus on it.
If I'm doing UWorld, I feel like it's a good indicator of my progress. Stuff I'm getting wrong there... I need to study.
(Yeah, I'm done with Kaplan)