Bit of both. If you were looking to build a cantenna(yagi), but bought a omni instead, then you aren't getting the same thing.. omni and yagi antennas are built for two different purposes.
Yagi has a higher gain, and is used for point-to-point links between two locations. Omni have a lower gain, and are used to cover a larger amount of area.
The example I used was a cell tower. 4 Omni antennas are generally used, one in each direction, to connect to subscribers. Each of these omnis has about a 90 degree yield, which allows them to spread the signal over a wide area. The Yagi on the cell tower has a much more focused signal and is used to connect to cell towers that are farther away.
If you were looking to build a point to point link and bought an omni, then notice that they are not the same thing, and your link will not be able to go as far due to the lower gain.