Considering you only have 8 units at the moment, you can do both geography and a science if you'd like! Geography is like a social science (meaning there is more discussion and debate), but it's completely up to you whether it's easier to do than the "mainstream" sciences. Generally though, people see geography and pretty straight forward and just a matter of practise and revision.
Choosing which science to do is completely to do with your goals and interests. Physics is more mathematically based, and compliments Mathematics Extension 1 well. Chemistry is considered the "base" science, meaning it is a prerequisite for the majority of science based university courses. Biology is more English based, and involves more writing and explaining through words than physics or chemistry. Earth and Environmental Science has very little mathematics, but is very beneficial for those who enjoy geology or mining. Senior Science is a more general science course, covering aspects of physics, chemistry, biology and EES.
In terms of scaling for all of these subjects, they go roughly in this order from best to worst;
Chemistry -> Physics -> Biology -> Geography -> Earth and Environmental Science -> Senior Science
Try and choose a subject based on how good you are and what you enjoy rather than who is teaching it. You're better off in a subject you're good at and enjoy with a bad teacher than in a subject you aren't good at and don't like with a good teacher. Besides, many people in similar situations find that teachers improve dramatically with their teaching in Year 11 and 12 because it is more serious. Just something to consider.
Good luck!
