Energy
Given the fact that all the new science required will eventually come to pass, the one constant seems to be the the massive quantity of energy required. This may be beyond our abilities forever.
Since I believe we will eventually go to the stars, I have come to believe that we will change the nature of time by extending our life span to a point that the distances involved at sub-light speeds will no longer be meaningful.
Given the two alternatives the problems of life extention are minor compared to space warping.