I've noticed some massage girls pretend to be Japanese, advertise as such, but are not. And a Japanese friend of mine suggests (not being politically correct) that Japanese women are a the top of some kind of asian fantasy status latter, probably because their higher economic reach (less and less true as China's economy expands), more worldly fashion (?), and the seemingly much higher volume of Japanese glamor and porn sites out there (China mainland is not a porn power house for obvious reasons).
At the other end, sadly, it seems asian women from poor countries are the most dominated, maligned, used and abused (ex. Asian Street Meat site). I hear that if you go to Thailand, say Bangkok (the best name for a city in this man's world) and scout for higher priced hookers, they are often not the native girls, they are Japanese, Taiwanese, Shanghai, etc. women slumming it in Bangkok or Phuket (also another great city name if you twist the pronounciation).
Seems like there is a pecking order, at least based on anecdotes.
I've noticed (apologies if I offend anyone), that Chinese MPA and SP are a bit more likely to be older (yes, I've got some slightly gray haired massages, just close my eyes and pretend). Chinese gals seem in my experience to charge a little less here in the greater Toronto market -- with a few exceptions. Basically, the more "fresh off the boat" (sorry, it's a term people use, I don't attach stigma to it) the more long hours and low pay these girls work. It's a generalization of course, there are always counter examples, I could be wrong.
For my tastes, I think all of them are beautiful, Korean, Chinese, Japanese, Thai, Cambodian, Vietnamese, Filippino, all the others. It happens to work out that in my massage parlour trolling for asian ladies, probably 60% have been Chinese, maybe a higher percentage. Only once did I meet a Japanese German mix girl who charged me $400 for an utterly awesome session. Viet girls I have not encountered too much, but they have always been fun when I did meet them. It would seem Korean and Thai are the next most common after Chinese, but I'm not sure. Anyway, like another poster said, you could write a great Stats Can or school thesis on "Ethnic composition and comparitive economic earnings of Toronto massage parlor workers". I promise to write a 100 page report on just that if only I can get funding to visit each GTA establishment a few times each for maximum services.