Our company is actually called
Dominate the GMAT, so perhaps there's the answer to your question

In all seriousness, what criteria are you looking for in terms of evaluating companies? More important than "number of users," in my opinion, is what the respective companies will actually be able to do for you. Are you looking for a large corporate player with name recognition, or a smaller boutique company that focuses on personalization and customer service? Do you want a more expensive classroom course in a fixed location near you, or do you want the flexibility and cost effectiveness that comes with online GMAT prep programs?
Based on your question about mobile apps, it sounds like online mobility is important to you as well. Most online programs are mobile-friendy these days, and with a course like ours for example, you can certainly watch the video lessons on-demand from your mobile device.
My encouragement would be to read testimonials from each of the major companies you're evaluating and talk personally with someone high up in the company or, ideally, the person who will actually be your instructor. Best of luck to you and don't hesitate to let me know how else I can help!