Recalling from what I was told when I went through the interview process at 18F (got to the point of the gigantic "dump your whole life in a massive form" bit,

Recalling from what I was told when I went through the interview process at 18F (got to the point of the gigantic "dump your whole life in a massive form" bit, right after the OPM hack was news, and decided not to do that), the positions are legally time-limited: it's a two-year contract which can be renewed once. So maximum of four years in the job, and then you're out. Also USDS is a bit more "do this mission" style job: basically you're being dropped into a federal agency to try to turn their tech story around. 18F is there to promote longer-term change in the model of how the federal government does technology.