The modes are both "event socket" and therefore quite scalable. The primary difference is in how the connection gets started.
With outbound event socket, there is always a call involved. A call comes in to FreeSWITCH and somewhere in the dialplan there is a call to the socket application. At that point FreeSWITCH makes an "outbound" connection to your application which is listening for connections on the IP:Port in question.
With inbound event socket, FreeSWITCH is the one sitting there waiting for your application to connect, i.e. an "inbound" socket connection from your program. Once your program is connected it can do pretty much whatever you want: show channels to see what's going on, originate to create new channels, fifo list, conference list, etc. etc. BTW, fs_cli is a great example of an inbound event socket application. fs_cli literally connects to FS via event socket.
Hope this helps.
-MC