What are the differences between the open assessment and the PSM-I assessment?

Hi,

I'm doing the PSM open assessments and, in the last 3 exams, have one or two fails only. So, I think I can do the PSM-I assessment and pass it but I can't stop thinking in the difference of the questions between the open assessment and the real assessment. Can anybody help me to decide if I'm prepared to pass the assessment?

Thanks in advance.