By main and advanced, I think you are referring to CHM1011 and CHM1051 respectively. Have a read of the links and pay attention to the Outcomes for each as well as Workload requirements and Assessment.
CHM1011:
http://monash.edu.au/pubs/2016handbooks/units/CHM1011.htmlCHM1051:
http://monash.edu.au/pubs/2016handbooks/units/CHM1051.htmlIn my opinion after some research, these units are similar in many aspects and the workload would not be significantly more in CHM1051 than in CHM1011. If you enjoy chemistry and laboratory work then CHM1051 will suit you well (also they recommend that you have a VCE Chemistry Study Score of 37 or above). Otherwise CHM1011 is just as good in equipping you with general chemistry knowledge.
Also, if you are undecided, I would choose CHM1051 and try it for a couple of weeks and then change units to CHM1011 before Census Day if you think it's too much work.