I find liquid eyeliner much harder to use; a slight shiver in your hand which would be negligible with pencil can produce a rather nasty gloop with liquid, and I find it much harder to make it symmetrical. Liquid can be used equally well, it's just more difficult to learn how to do so.
Torrain Also, after you reach a certain age (cough, cough), liquid liner follows every bloody line & crease in your eye lids, while nice, smudgey pencil is very forgiving to the older eye. My fav is Revlon Colorstay Black; it's soft, but doesn't smear all over, gives nice deep color without having to hammer it into your eye. I also set it with black shadow.