That is a hard nuance to pinpoint. I always have one GM-PC (I want to play in my own games!!) and several NPCs. I think for me the difference is my attachment to them and how I move the plot with them. I try to make my GM-PCs not so central to the plot so they don't/can't run amok over the other players (see my Arkady story for why this is!), the NPCs typically are villains or say the PC's employer and I use them more heavily to move the plot. But then the lines are blurred sometimes.