For performance downwind halyards, I agree with the choice of Dyneema (or Spectra). 8mm / 5/16" is perfect. Dyneema is UV stable enough to be stripped to reduce weight aloft, but at 8mm, I'm happier with the extra abrasion resistance. Before my current Endurabraid spin halyard, I had stripped 9mm Dyneema which wasn't bad either, but I prefer using 8mm as the method to keep the weight aloft low, and 9mm is harder to find.
As I know it, Spectra (a trademark of Honeywell) is very very close to the same thing as Dyneema (a trademark of DSM), but is not nearly as similar to Vectran which is an aramid, which is much more like Kevlar and Technora.
For performance upwind halyards, Vectran and Technora are excellent. I have 10mm / 3/8" V-100 for my primary genoa halyard. I don't think that I would buy another one that thick for a J/30, but would really prefer 9mm / 23/64" for creep reduction and "feel" over the 8mm size. While it is listed on neropes.com, I couldn't find it. I also use Yale Aracom-T Technora that seems to be about 8.5mm and like it. According to my local rigging shop, it is a bit more dimensionally stable than Vectran, but they seem pretty interchangeable to me.
