Depends on the attraction really. I know that single rider on Soarin over California can either be a blessing or a curse. I've used single rider and boarded right away and other times you end up waiting for quite a while before you get to board.
For most attractions, if I was alone, I'd prefer Single Rider. But for Soarin', I think FastPass is better. You're grouped with everyone else and are more likely to get a decent seat. Single Flyers on Soarin' are like umm... people on standby at the airport, getting space-available seating, which isn't always the best.
The true answer depends on the ride. I am always amazed when there is a 120 minute wait for Splash Mountain (like there was the weekend before last), and I get into a log using Single Rider in LESS THAN A MINUTE (no kidding)!
Same thing yesterday at California Screamin', and Grizzly River Rapids. Right on the ride, almost no waiting.
However, Soarin', at least the way it operates, is not the same -- it's better to use FastPass than use single rider for that one particular ride.
Single Rider is a godsend for Splash Mountain. I only wish they did single rider for Space Mountain. The seating is no different than Screamin'. They could just have you come in thru the exit. (Just like Splash)
Star Tours could easily handle a single rider queue. They could just have the single riders go up the wheelchair entrance. There is usually always an empty seat or two per cabin. But maybe its line doesn't get long enough to justify a single rider option, I dunno...
I think single rider is the best because you can ride as much as you want and there usually is NO line!
I dunno about this, often times when I want to use single rider for Splash Mountain the line can last as long as the stand-by line. Depends on the CM. So I never depend on it, fastpass is waaaaaay more reliable for getting on rides quickly.