I remember the days when I'd get a standard servo and link the red wire to the + on the speedo wire.
I think it's personal preference, I always go for a KO with metal gears, I don't really mind if it's digital or not. I'm probably buying the same servo with a different name plate.
However if you really want to know the difference here it is:
http://www.futaba-rc.com/servos/digitalservos.pdf