For me they're not the same thing. Take the FMS weekend for example. I definitely got my share of good girl spankings, and I think of them as good girl spankings because they're definitely not punishment, not even bad girl spankings or mock BG, and the aim of the Tops that I played with was to make me feel GOOD, by way of providing me with the sensations that I enjoy. During those spankings, I was completely relaxed and enjoyed myself thoroughly, yet wasn't turned on in the least.
Erotic spankings require me to be in a certain mood. The most important factor is the person that I'm playing with. I have to be able to feel comfortable, familiar and intimate enough with the person I'm playing with to be in the mood to feel sexy and turned on, which is usually the result of an erotic spanking. Usually this person is a romantic partner, or someone I'm sexually involved with. When I am being spanked by someone whom I have only met 5 or 10 minutes with at a spanking meet or club, I definitely don't consider those spankings erotic, especially if they're someone I can't imagine myself "being" with, if that makes any sense. But those spankings can most definitely be GG spankings, especially if they are given long and leisurely, with just the right amount of sting, and after a good warm up.
