I use both, but am friends with the folks who run Dopplr. Personally Tripit is more like the mile tracking / flight focused tool that will appeal to the FT crowd. Dopplr is more about finding friends who have overlapping schedules, knowing what your friends like in a new city, etc...

Trippit is much more focused on your flight, your tickets, miles, etc... Personally i get more out of dopplr, but your mileage may vary.

-rabble