Self-paced online classes: Knewton, Manhattan, or Veritas?

I'm looking at taking the online GMAT classes, since there aren't classes offered near where I go to school. And I'm deciding between Knewton, Manhattan GMAT, and Veritas. I've done some research of all three prep classes and read some threads here as well, but most of the reviews are about in-classroom courses. Personally I'm leaning towards Manhattan GMAT (Prep Plus), but it's the most expensive one among all three and I get access to the online materials only for 6 months (I'm still a college student, so I have plenty time and would like to take advantage of that). And I'm not really familiar with Knewton online self-study classes. Aside from the generic company advisements which I can get from their websites easily, anybody that has taken any of these online self-study classes can provide some personal experience with pros and cons? Any input would be greatly appreciated!