I see a lot of people that use both terms, but it seems there's a trend towards more medically-oriented practices using the "aesthetician" spelling. Have you noticed too, or am I just imagining things? :-) Really, I don't think it matters, but I guess words only mean whatever people make them mean, so it's an interesting discussion.
I have seen aesthetician used more for medical estheticians but like abram stated, I don't think it exactly matters. I've been seeing a lot of "skin care professional" "skin expert" "skin therapist" and such too used