What are the similarities and differences between slaves of the Americas and convicts sent to Australia?

In both cases, the British Empire shipped thousands of people across the ocean by force and without their consent, where they ended up establishing a new population on a new continent. In both cases, the process was violent, oppressive, and dangerous to the people being transported.

But there are some vital differences between the two.

Perhaps the most important is how the people were selected.

Africans who were sold into slavery were chosen first of all for their race and skin color, and only secondarily as convicted criminals or prisoners of war (as these were the Africans that were most likely to be sold into slavery by the existing African societies).

Prisoners sent to Australia were all convicted criminals, most of them as far as we can tell given legally-valid trials in the British courts of law. (Whether these trials were fair or the laws they were based on were just is a more difficult question.) The Africans who were sold as slaves could have otherwise been free individuals; the Brits who were transported to Australia would have otherwise been imprisoned or executed in Britain.

Another key difference is what happened to them once they arrived.

Africans sold into slavery were immediately put to work, predominantly on plantations, given only the barest minimum of food and housing, and offered no pay and no chance of quitting or working anywhere else. Many were beaten. Families were forced apart and people were "bred"---that is to say, raped and their children sold---as if they were livestock animals.

Prisoners sent to Australia, on the other hand, were given surprisingly free rein; the British government apparently reasoned that once they were so far away, they could do no harm, so it was easier to simply leave them alone under minimal guard and supervision.

Many prisoners were made to work under poor conditions for low wages, but this was not so different from free workers at the time, and other prisoners even became wealthy and powerful in Australian society as business owners or political leaders. Most of the prisoners were separated from their families (as prisoners typically are), but not all; and the prisoners included both men and women, and many started new families upon arrival. Likewise, many of the soldiers sent to provide supervision brought their families along and started a new life in Australia.