Install PostgreSQL using source code or using apt-get?
If you install from apt-get you will be able to use apt-get upgrade option later and other apt-get build in options (available from Ubuntu). 
Furthermore apt-get installs binaries and manages their versions. You just install and nothing more.

Installing from source gives you the ability to do a more detailed installation. (You can customize the build and installation process with command line options connected to ./configure PARAMETERS LIST.) It's useful if you need a feature that can be enabled only by compiling the package yourself. Next thing is that installation from source may not be visible to apt-get commands so you may not be able to use them. 

But if you need to install Postgres stable version without any specialized functionalities use apt-get option.