Somewhat different reasons.

Peter the Great really wanted to emulate a beautiful European city – specifically the watery city of Amsterdam, filled with canals and along a riverbank. The Neva delta was very pretty to him, a lot of waterways and small islands and flat shores, so as Tsar he ordered his new city to be built there in the model of European cities. Supposedly, he saw the auspicious sign of an eagle as well while scouting the area.

The American Congress wanted to reach a compromise between the competing interests of Southern and Northern states, as well as create a capital distinct from the states that could maintain its own safety. (The previous Congress building in Philadelphia had been besieged by protesting war veterans, and the Pennsylvania government did nothing to stop them.)

In exchange for building the capital between the two northern-most slave states, Maryland and Virginia, the Southern states agreed to take on a share of the North's war debts. Maryland and Virginia authorized a tract of land to be given to the federal government along the Potomac River, though it was President Washington who selected the specific location along the river.

He chose a site next to the already-existing Virginian city of Alexandria, in which his family owned a fair bit of property, and which also happened to be just a few miles up-river from his home.