A personal room is like your office on the web. It's a personal space that's always in the same location. People who know the address of your personal room can drop in any time. You can also hold meetings there, and you're the host unless you specify an alternate. It's best for impromptu sessions or small meetings where you let everybody attending know to use your personal room.
A scheduled meeting exists for a specific place and time. You set the time, list of people to invite, host and meeting settings. This is best for larger meetings, meetings where you want to use customized settings, or meetings you're setting up for somebody else.