I absolutely love NextStep, and my answer may be superficial. But, I like their exams because they have the same layout as the AAMC materials. I need consistency when I study, and just knowing that the purple bar on the NextStep practice exams will be similar to the same purple bar on the top of the real AAMC MCAT is comforting.
So far I have taken NS exams 1-5, and I am taking #6 tomorrow morning. But, one thing I did realize is that there passages seem to be a whole lot more involved compared to EK and to AAMC's. I realized that with each Chem/Phys and Bio/Biochem passage on NS comes many diagrams, many tables, and many graphs. Whereas on the AAMC section packs may only have one of each (table, picture, graph) NS will have multiple. I can't say for sure if this means anything for our scores... but in my experience, looking at a NS passage at first glance makes me hold my breath and buckle down, and looking at an AAMC practice passage looks a whole lot do-able in comparison.
Comparing EK and NS, I like the post-exam feedback from NS, and I do think I gained more from reviewing a NS exam, than from the EK exam. NS provides stats on the percentages of test takers that got the question right/wrong much like the MPREP Q's of the day. They also provide a section, topic, and skills breakdown at the end of each exam. But I would hate to be the voice of reason, and I would hate to push you to go with a study resource that doesn't fit your needs! I promise I'll jump off this thread and head back to studying... good luck, good luck, good luck!