I just got Kaplan yesterday. I've been doing Uworld all along. Really, I don't think it matters too much. If I had to absolutely choose which one to pick to do first, it'd be Kaplan now that I have it. Not because of validity, but the first aid references and what not make it much easier to review early on.
I plan on resetting my uworld and cranking through that for most of my independent study time. After 8+ months of having it, I have a difficult time believing that I will skew my answers too much because of knowing the question.
Really, it is all about doing as many questions as I can and seeing the explanations so many times that I'm sick of it...and then seeing them some more.