Around here we have mostly double track fire roads and a few legal single track sections of the Shawnee National forest. I also make a couple of pilgrimages each year to TSALI, a single track mecca on the South side of the Smokey Mountains in North Carolina.
I have said it before, if all I had to ride was pavement, I would not ride. Now, given a choice between Single or Double, I will choose single track every time. I ride more double, but only because the inventory of single is so slim around here.
Just like whitewater, the scale of trails is different in various parts of the country. I can only imagine what Fred has in the mountains of CA. His fire roads would probably scare the hell out of me. And Funrover would probably laugh at what I have for Single track to ride.
Where I think it is hard to compare single to double is when single gets technical. At that point, the speed that Fred goes up or down a fire road can't be compared to the 5mph that Funrover might be doing downhill as his back tire slides down a snow covered trail, or he has to do a track stand to prepare to drop off a rock to continue the trail a few feet below. Both Fred and Funrover need skills, just different ones.