This question came in yesterday:
"Hi I am looking to buy two lightspheres for my 2 Canon 580EX II flashes that are mounted to light stands for quick off camera flash portrait setups. Would you recommend the clear or the cloud for this type of setup or would the whaletails be better suited for this. Thank you" - Tony
The whaletails give the most versatility. That is what I prefer for studio use. If you go with lightspheres, I would use the Cloud as your main light. Another Cloud or Clear is then your background or fill light.
One thing -- the Gary Fong website has an error. It lists the size 4 for the 580X-II. Actually, that is for the older , and smaller 580EX. Most people are getting size 2 for the mark II.
Gary just introduced a lightsphere-universal (has a one-size-fits-all mount), at PhotoExpo+, last month, in NY. It is only produced as a Cloud, but it will fit a variety of flash models. Unfortunately, he sold out at the show and won't have them in his eStore for a couple of weeks.
Whaletails are in stock. If you go that route, get a Studio for your main light, and a Reporter for the fill/background.