USDS and 18F serve in pretty different roles. USDS often comes in when there's a crisis or a high presidential priority, because the organization sits in the office of the President. 18F is located in the General Services Administration and is especially strong when looking at the medium- to long-term goals of an organization. I'd also say that 18F thinks more about culture change than many parts of the USDS.

They also have different funding mechanisms that lead to different operating models. USDS funding comes out of a budget called ITOR and we can represent a source of "free" engineers for an agency. 18F operates on a cost-recoverable model where an agency is paying for help or services.

There are other, smaller differences too. For example, it's typically easier to work remotely for 18F than for USDS.

From a distance, the organizations both look like "tech in government," but I think they operate better separately. I have enormous respect for the folks in 18F, and the two orgs have collaborated on things like the US Web Design Standards: https://standards.usa.gov/