The debate you are having is between Learning and Training.
If you use the Qbanks to Learn, then the goal is really to read the explanations, link them to First Aid, and you don't care about getting the answers wrong. You can't possibly get the answer right if you haven't learned the content yet. This strategy is superior if done during your second year in medical school or at the beginning of your Step studying. It is the "first-pass" at the Step material and you will still be synthesizing the information. For learning, I recommend Kaplan qbank as the explanations are superior, they have ReKap, references to First Aid, and the questions are harder.
If you use the Qbanks to Train, then the goal is really to ignore the explanations but to get used to the hinge-type question, get used to the grueling pace of the exam, and to test your knowledge. This can also be useful to assess where you are at currently. As you said, NBMEs sort of do this. For trianing, I recommend UWorld QBank as the questions and interface are almost identical to the software you use on the real deal.
If you have to choose only one Qbank, I would pick Uworld. Just because Kaplan is better at teaching doesn't mean Uworld doesn't teach. The emphasis must be on Learning over Training. And again, you have other resources to assess your performance, that is, to train.