I just signed up for knewton and their course is so awesome. The content is much better than kaplan and very close to what real GMAT tests. UI is very good too. Every question is tagged and hyper-linked to its underlying concept so that you can go check the concept if you got the question wrong.
The content at Kaplan seems to be outdated. Math questions are very easy there by artificially increase the score in practice tests. Also, some of the important concepts like data interpretation are not covered at Kaplan.
Finally, the price is not comparable by any means.
If anyone needs knewton referral, please contact me and save yourself $150. rabi280 at gmail dot com.