I've thought about this issue for a longer time but I'm not able to make a decision. In my opinion the environment has the same level of importance as human beings - for the very reason that we are reliant on it. What gives us the right to judge the welfare of human beings as more significant than the welfare of the nature and the animals? Although it could sound naive, both - environment and human rights - should be protected in the same way!
Nevertheless I like Jonathan's question about the influence of the price on our decision-making. I would bet that most of the people would choose the cheaper oil - whether it has a bad impact on the environment or human rights. In the end, cash is king - unfortunately...
Pretty easy choice for me, harm to the environment is the lesser evil compared to harm to humans. Or, to consider the longer term environmental impact, potential future harm to humans is the lesser evil compared to real current harm to humans.
Perhaps another interesting question is how much more I would be prepared to pay for the lesser evil. If gasoline refined from tarsands cost $1.60 per liter compared to $1.25 for oil from countries that perpetuate human rights abuses, would I still choose the former? What about $2 per liter?
In my view, both are detestable. However, in a selfish way, I would rather see some other country be inflicted with human rights violations than my own country.