They have treatments that can generally prolong HIV patients lives a LONG time and prevent them from getting full fledged AIDS for quite a while.
Many Cancers can be removed or treated, and survivabillity is already pretty high for the most part. (Though this is the number one desired medical cure IMO, by a longshot)
and Herpes isn't fatal. There are meds out there that pretty much prevent 90% of the occourances. Sure it's a hastle for those who have it, but they can still live a mostly normal life.
Room Temperature Superconductors, could in theory, completely remove our dependance on fossil fuels. It would basicly allow for limitless generation of cheap electricity and energy. This would be very good for the earth, and in the long run (by long run, I mean LONNNNNG Run) .. It could potentially help humans avoid future wars for scarce fossil fuel resources, as well as prevent a lot of global warming, and preserve our "way of life" and improve quality of life for all, for generations to come.
This is a hard decision, however, I would opt for the superconductor. It would be better for the planet as a whole in the long run. I also believe it would be better for humanity. I know I'd like to artificially extend the duration of my life, as well as the lifes of all those whom I love, however, Even if Cancer was cured, people would still die from Heart complications, falling down the stairs and breaking their hip and then being stuck there for 15 days and starving to death, etc. It would add a LOT of years to many peoples lives, but it would mostly add years to the lives of those who's candles are already fading. If Aging as a whole could be stopped, and people would no longer succumb to organs failing because they just can't hack it any more, etc. Then I believe that would be a bigger and better discovery for humanity, however, given the two options above, I still believe the superconductor is the best between the two.
I guess I'd rather see us Move Forward then Live longer.
Our Children, Our GrandChildren, and all those further down the line, will carry a little bit of our genetic code, as well as the teachings that we give in life to them. Even though we as mere mortal humans may become deceased due to a cancer, the best of us will live on in our descendants.
Cliffs: The Superconductor, but it's close.
EDIT:
A question for the OP ...
Assuming we get our superconductor in 5 years, and within 10 years we have prototype power stations using the superconductors, and within 20 years, we basicly have unlimmited power ..... Why could we not funnel all of the resources that went into the superconductor, as well as all the resources that were previously spent on harvesting, transporting, etc fossil fuels, towards the development of the miracle cures?
It would make much more sense to tackle something that would save resources, so we can then use more resources for the second challenge, would it not?