tough descision, but i went with injury.
Age would be awesome as you would be able to live through the earth's future and witness new technologies, the discovery of the first extra-terrestrial life, and you could work on things that would normally require a series of lifetimes to do.
However, imunity to injury would be great. if you never could get hurt you could stand up to anyone without fear that you would get beat, and i'm talking big groups of people like the talliban, and fox news... You could literally become a super hero if you wanted, though i don't think id go for the costume and the famouse lifestyle, just hang around london fighting crime in the darkness. would be awesome if i learnt ninjitsu as well, i'd be a real life ninja that couldn't die, hehehe.
So i recon injury would be better, though both would be nice, but only if i could injure myself. longevity would only be descent for a few hundred or maybe thousand years =P