A very interesting question! My first reaction is that there is at least one other important variable here; in addition to RL-experience, I think our attitude to pain has a lot to say for how we prefer our spanking stories. For me spanking is primarily about mental processes; about doing wrong, being found out, getting punished, accepting one's guilt, being forgiven, and the pain is only important to guarantee that this process is genuine and believable. Other people may have a more "physical" approach to spanking where pain is of greater importance in itself.
I have some, but not much, RL experience and definitely in the "mild" category. I can still appreciate "hard" stories as long as they are either playful or make psychological sense. I usually don't like stories where blatant sadism is presented without reservation (and even with relish), and I particularly dislike them when the victims are underage and in a situation they cannot escape (such as being brutalized by their own parents). Still, I try to remind myself that this is fiction, and that the license we willingly give to authors of crime and spy stories, should also be extended to writers of spanking stories!
What actually bothers me the most, is when otherwise fine stories are ruined (for me at least!) by excessive cruelty in the spanking scenes. To some extent this is a matter of taste, but not totally — when otherwise loving parents in a realistically written story turn into brutal beast the moment they get a hairbrush in their hands, something is usually wrong with the writing — probably that the author has switched from "public writer"-mode to "private fantasy"-mode.
