Is Doing NBMEs on iMD Enough, or Should I Buy Them Officially?

Hi everyone, I’m preparing for Step 2 CK and wanted to ask for advice from people who have already taken the exam. I purchased and completed UWorld through the official UWorld subscription, but I’ve been doing my NBME forms through the iMD app rather than purchasing them from the official NBME website.
For those who have taken Step 2 CK, do you think doing the NBMEs through iMD is essentially the same for preparation, or is there an important benefit to taking them through the official NBME website? I’m especially concerned about whether using iMD could affect how accurately I judge my readiness for the real exam.
I currently have NBME 15 and 16 left. Would you recommend purchasing those two officially, or is doing them through iMD sufficient? I’d really appreciate hearing from people who have actually taken Step 2 CK and what worked for you. Thanks!