To me, testing on prison inmates is totally unethical. Just because someone is convicted of a crime does not mean they lose their humanity and become human guinea pigs. Volunteer testing is unethical too as it puts pressure on poor people to become said human guinea pigs if there is money involved. I suppose you could make an argument that it's unethical to test an "innocent" animal over a "guilty" human, but I think it's much "cleaner" to draw the line at human / non-human than to potentially make testing a race, gender, religion, sexuality, etc. issue based on who gets experimented on. I do think it's worth testing on animals to save human lives because would you be okay if a loved one dies because people thought it is unethical to test a potentially life-saving cure on rats, guinea pigs, or even chimpanzees? Ethics are highly opinion based and there are cases to made both ways, so ultimately it comes down to what a majority of people are okay with and currently that means testing on animals.