Hi gmatarbitrage,
There are lots of different GMAT resources available for you to study with, so it might be daunting to try to put together the best combination for YOU. However you choose to study, you should place an emphasis on 'realism' - not just in the quality of the materials, but in the form that those materials take. You're going to end up taking the GMAT on a desktop workstation in a computer lab, so making sure that you train for the physical, mental and psychological aspects of TEST DAY is a MUST.
Test Takers who use a 'book heavy' study approach often get 'stuck' at a particular scoring level; in that same way, using an 'app-heavy' study approach would likely create the same problem for most Test Takers. This is not to say that using books or apps is a bad idea - they can both be quite helpful during your studies, but neither is likely to be enough to help you to hit your score goals.
GMAT assassins aren't born, they're made,
Rich