As to the second point, the channels may differ, but a city that's affluent and embraces technology should see an proportionate increase on both categories, housing and erotic services. And LA has a great climate for "hollering at cars", while Boston should probably have the higher rate due to weather.
I'm certainly not going to argue that this analysis is anywhere near perfect, but I think you'd agree that Boston at 4% and LA at 24% is probably not just a matter of people moving more frequently.
I'm completely open to a better metric. But as I said in the reply above, it's not easy to determine just how many people in an area use craigslist.
That being said, I'm not sure the two categories have such a hard seperation between the users. People on craigslist who use it for finding prostitutes probably also use it to find housing and vice versa.
The second is that Craigslist is not the only channel for prostitution. There are high-end channels such as those used by Eliot Spitzer which may be more common in affluent cities, any "brothel"-type establishments that exist, and of course women on the street hollering at cars (the ability to do this may be severely hampered in colder climates).