Jay Fox has been a prolific, thoughtful writer at the Immortality Institute forums for some time now. He's also involved with the M Prize volunteer group and efforts to start a Methuselah Fly Prize. After much arm-twisting, he's now blogging on healthy life extension at Longevity First. Add it to your bookmarks.
Perhaps the simplest and least controversial reason to pursue longevity first is that the other technology can wait.
I dream about space travel and mining the asteroid belt and colonizing the moons of Saturn just as much as the next technophile. But, I can wait a few decades to see those dreams achieved if it means that we can prevent a few hundred million people from dying who otherwise might not have to.
More to the point, I want to see those dreams of space exploration come true in my lifetime, and that merits spending vast resources on space technology. However, with an extended healthy life span, I can wait. If by curing aging, we can give people in their 40's and younger a pretty decent chance of living to 150, then we can all wait a few decades to invest those vast resources in space technology (or fusion power, etc.), and instead invest those vast resources into longevity research. In other words, we have so little to lose, and so much to gain, by making longevity research our highest priority.