I haven't used it myself, but it looks like the goals are different. TripIt is meant for trip planning for a single person and organizing travel and stay mainly. Calenday is meant for cross-user collaboration of trips and getting consensus on activities between multiple people.
I use TripIt for multi-person travel itineraries. It does allow for sharing and multiple contributors. It is not necessarily for getting consensus on activities, but for centralizing all of your confirmed plans in one place.
