The 'face sitting', or to describe it more accurately, face kneeling, is okay for fun and play, but is not particularly effective for cunnilingus because it does not allow the man's head enough mobility to access all her important little places. The same thing goes for the '69' position.
The optimal position is with the woman on her back, with legs spread and relaxed. Even better is to have a pillow under her ass so that her pelvis is tilted upwards, raising her genitals to a better angle. This way, an effective tongue bath can be delivered to her entire underside, and the man can focus on her clitoris or other areas as he pleases because his head can move freely. He can also use his hands so that she feels better connected to him than just his tongue. My wife loves it when I place a hand on her belly and lightly stroke it, or stroke her thighs, especially along the insides but also down the outsides. Fingers can gently be inserted into her vagina or anus as she wishes, all while keeping up my tongue action on her clitoris.
As this seems to be turning into DrZ's Sex Tips for Boys, I'll say that most importantly, take your time!
A variation that is a little easier on the man's neck is for the women to lay on her back with her ass on the end of the bed. You then can kneel between her legs and have excellent access to her genitals. Again, the man's hands can come into play and she will feel more connected to him.