In preliminary economics, you study things like financial and labour markets.
HSC economics explores topics like the global economy and Australia's place in it, as well as economic issues.
In preliminary business studies, you study things like the nature of business as well as management.
HSC business studies explores 4 key business functions: finance, marketing, operations and HRM.
I can't think of many similarities between the 2 subjects, but in the business HSC course the finance chapter includes content that might also be taught in economics (such as exchange rates and economic outlook).
Business studies is a fairly simple subject but there's a quite a lot of writing. If you have an interest in business and are willing to study for it, getting good marks won't be hard.
I don't know much about how hard economics is unfortunately because I haven't done it.