First of all, it shouldn't matter what source you're prepping from. Yes, some are "better" than others, but if you're doing practice questions the right way, they don't need to be the "best" questions in order to give you a significant benefit. More on this "right way" below.
One of the errors a lot of students make -- and this is, for obvious reasons, especially prevalent among non-native English speakers -- is to basically memorize the wording of the practice questions in whatever book or course they're using. Remember that the GMAC problems are copyrighted, so none of us -- Knewton, MGMAT, Kaplan, Veritas, BTG, etc. -- can use the same style of wording. Furthermore, even the officially-released GMAT questions do not cover every style of wording and are all slightly-out-of-date questions, meaning they've reworded new questions that none of us, myself included, have ever seen. GMAT skill is NOT about memorizing the exact words and phrases that tip you off to right or wrong answers, and most elimination techniques are built upon these kinds of tricks (incidentally, these do work much better for the SAT and LSAT, which are updated less frequently than the GMAT or GRE). Instead, to do well on the GMAT, the real skill you need is the skill to, given a totally random out-of-nowhere problem that you've never seen before, interpret what they're "really" asking and translate it into a problem you HAVE seen before. So, unless you're doing this translation regularly and honing this skill, then even getting 100% of the MGMAT or OG or Knewton or whatever questions correct will not necessarily give you a great score on the GMAT.
The right way to practice for the GMAT is to remember that it's practice. Think of all the questions you'll do in anticipation of the test. Hundreds, maybe thousands of loose practice problems in Math and Verbal; at least a half-dozen practice essays; and 5-10 CATs, at least, for most of us, so that's another few hundred questions. Ask yourself, how many of these questions, the thousands you'll do before test day, actually affect your final score? Answer: zero. The only questions that matter are the 80 (2 essays, 37 math, 41 verbal) that you see on test day. Everything else is just practice.
As you're doing questions, you never should really care whether you got it right or wrong. If you get every single practice question wrong and then do great on the real 80, that's a great GMAT score and you're happy. If you get every single practice question right and then do terribly on the real 80, that's a terrible GMAT score and you're upset. Admittedly, those two scenarios are unlikely, but the point is that ONLY the real 80 matter.
What this means is that, on every practice question, don't ask yourself whether you got it wrong or not. Ask yourself the following when you start every question:
1) What kind of question is this?
2) What is the correct approach?
And the following after you're reviewing it -- whether you got it right or wrong ask the same questions:
3) Did I identify the question type correctly?
4) Did I quickly decide on the right approach?
5) What should I do next time I see this question?
That final question is by far the most important. If you get a question right, it matters HOW you got it right. If you got it right "the wrong way," then that is worthless to you because it won't be repeatable on a differently-worded question on the real test. If you got it right "the right way," you want to figure out exactly what is worth taking away from it and repeating. If you got it wrong, you probably need to figure out what you should have done differently and try that next time. If you got it wrong because of some silly error but can answer all 5 questions confidently, then that's great! Who cares that you got it wrong? Business schools will never know!
So, keep practicing practice questions, regardless the source, but stop thinking about how many you get right: think about your methodology, about the repeatability of your approaches, and about answering those 5 questions. This will set you on the path towards being able to figure out any crazy question the GMAT throws at you on test day, whether you've seen that exact thing before or not, and will keep you focused on the only thing that matters: the real 80.