I think that they kinda target different audiences - or, at least, they work best for different types of people. The GMAT Pill explains things pretty clearly, and goes through several different types of questions in a detailed manner. This makes it useful for learning how to really understand each question, and I feel that this is particularly apt for someone who's already got a reasonable grasp of the subject matter, but wants to push their score up even further.
MGMAT, on the other hand, has by all accounts an excellent set of practice questions. How is it working out for you so far? Have you tried any practice tests? I'd suggest finishing up that final review of yours, as you mentioned. After that, if you run through a few practice tests and seem to be hitting a wall, then you should think about picking up the GMAT Pill. I'd say that anyone scoring between maybe 600-720ish or so stands to reap the most benefit from the program.
Of course, if you finish practising with the
MGMAT resources and your mocks net you a score of 760+, then you could just head straight for the actual GMAT. The GMAT Pill's great and all, but even it can't fix what isn't broken.