The simulator helps in assessing your preparedness for the exam and identifying your weak areas. Many times, even experienced programmers do not do well in the SCJP exam because they lack in the right kind of practice in certain topics. So the simulator is basically a tool for self study.
Online Training gives you the benefit of continuous guidance and feedback from the author of the simulator. The exam concepts are explained to you via interactive sessions in a virtual classroom, and you get to clear your queries instantly.