I've used both. I had tripit-pro through amex, and when they cancelled it, I decided I would try kayak for a few trips. Here is a summary: both about equal in submitting the trip data and understanding it and putting it into an understandable form (you basically just email the flights/hotels etc confirmations) - tripit probably has a slight edge as it automatically scans my gmail account. But where tripit pro shined (and I reluctantantly forked over the annual fee) was keeping me up to date on flight delays/changes. Tripit pro seemed to txt me before either kayak had figured it out, and many times even before the airline sent me info about delays/changes. Seems like these days that almost all flights are delayed (and more so during the winter due to the weather), so that is important to me. I also like the alternate flight info, if a flight is really delayed or cancelled.

So, if time is important to you, then go with tripit. Cheers.