I just started using Pastest and I like it so far. What I do is use it during the first few weeks of my block (while I'm still learning the material) then the last two weeks before exam I do USMLERx for the same organ system. If you think you can can get through Kaplan and uworld completely then maybe invest into USMLERx. If you probably won't get through Kaplan before starting uworld, then it's best to just use Pastest when you don't want to bomb your Kaplan average just yet. At least that's how I do it lol.