I agree they are different, but I disagree that o/l is fantasy and f2f is reality. I think the amount of fantasy v. reality in ANY relationship varies depending on the persons involved. Surely you have had friends whose perception of their f2f significant other was largely based on their fantasy of what they wanted the person to be or the fantasy the f2f person projected, whether intentionally or subconsciously. Moreover, it is wrong to assume that all online are merely roleplayers.
As to the D/S in particular, again what matters is the compatibility. Some subs need to be grabbed by the hair occasionally to feel submissive - online obviously wouldn't suit that sort of dom. Others get their pleasure from voluntarily giving up control - online is more likely to work.
Neither is better or worse, one just suits some people and the other suits other people.
fantassy