So if I had to decide, what criteria would I use? It's tempting to put my finger on the scale (for the pipeline, as the outcome that angers fewer people), but I'm trying hard to resist that temptation. The biggest reason for the pipeline is the jobs it would provide. With unemployment high and not many new industries developing, this type of project is too rare an opportunity--new development in sector that has a solid future and adds net earnings to the economy. (I assume federal and state governments aren't subsidizing this pipeline, but plan to reap net revenue gains. That better be so.)
On the other side are environmental considerations. The entire project is not doing the earth any favors. Extraction and refinement of this dirty oil will cause a higher level of pollution than many energy sources, even many other carbon energy sources. If this was a question on whether or not to develop this resource, the answer would probably be against development. However, it has already been developed, due to choices made by Canadians, not us. Also, this is not currently the dirtiest oil, so there is the likelihood of net environmental benefit. We could choose to take the position that we won't participate in such a polluting enterprise--that would be principled stand for those who value the environment far above other considerations.
Since the US doesn't control the development of the tar sands, and since it isn't the dirtiest oil currently being used, the environmental arguments aren't a slam-dunk for me. The US control is only over whether or not to build the pipeline. So we should know what the dangers of the pipeline are, and we should minimize them. However, I'm not completely comfortable with this position. The more people are involved in the carbon energy economy, the less incentive we have to make incremental changes to cleaner energy. I may be making the mistake of valuing political peace, expediency, and jobs over the global environment, and that may very well be shortsighted and cowardly.
Other sources on environmental impact: