One similarity is that both the Australian Constitution and the USA is that both contain express rights which are entrenched in the constitution meaning they can only be changed through a successful referendum. However a difference is that the bill of rights contains many more express rights, for example the freedom of speech, whereas there are only 5 in the Australian Constitution for example the right to trial by jury. Further they are somewhat harder to change in the USA as the referendum process is more complicated and lengthy.
A second similarity is that both approaches to protecting rights contain structural protections which indirectly protect the rights of the people by preventing a misuse or abuse of power by any one government body. However a difference is that Australia has the principles of representative government, responsible government and the separation of powers whereas the USA does not have a principle of responsible government. A further difference here is that in Australia there is some overlap between the separation of powers in the legislative and executive powers whereas in the USA there is no overlap because the president is independent from congress. A third similarity is that there have been implied rights found to be suggesting within the words of the constitution. In Australia it is the right to freedom of political communication whereas in the USA it is the right to privacy.
That is what i would write for 6 marks as it has 3 similarities and 3 (actually 4) differences. If it was 4 marks i would leave out the referendum bit and the implied rights bit.
I hope this helps a bit