I tried knewton and it's an excellent product if you love to learn from recorded video lectures. It is also a cheap option if you need to study for a long time, like up to one year. If you are sure you can get the money back guarantee AND get a high score, I'd recommend knewton. Check carefully though - some forum posts suggest it's nearly impossible to get your money back since knewton's terms require you to fulfill a ton of requirements to be eligible for it, not just to get a low score. E.g. if you have just 3 weeks you have little chance of completing all of the work sets and so won't have any guarantee from knewton. Again - it can be ideal for a very long study period if you're sure you can score very high in their diagnostic test and even higher in the real thing.
If you're not the type that needs a human face in a video in order to understand the explanations, MasterGMAT is for you - it's interactive and really smart actually. It is expensive no doubt, at $800 per year, but the weekly plans are affordable.
In any case, my suggestion is that you read reviews about both, understand the pro and cons of each, and only then make up your mind. Check out:
www.google.com/search?q=master+gmat+reviews (4.92)
www.google.com/search?q=knewton+reviews (4.57)
Good luck!