At the moment, I am using both. I like how Uworld explained the problems and also you can find the calculator steps if the question requires.
Kaplan, the questions are good as well, also you can print all the questions (you can create a test then print it). I have registered for December exam which is last paper based, therefore using Kaplan, printing out and practicing is very useful.
However, Kaplan`s explanations and solutions are not enough and clear in my opinion.
I just combined them. If you had a chance to have alook, tell me your ideas as well!