I would say yes there is a moral difference. I'm just guessing/assuming here, but here's my take on things ...
The Streetwalker is probably not clean (knowing they are sick and intentionally spreading disease is rather immoral IMO.) Also, the likelyhood that they are using the income to better themselves (as in, for a productive reason, such as pay for college) is very slim. Most likely they are just addicted to some type of drug and looking for a quick way to get a fix. As long as they just hurt themselses, I don't see it as being really immoral.
The "WILF" types are much more likely to use that income to better themselves, be it through education, or through investing. They are probably clean, or at least a much larger percentage of them are clean, they get tested regularly to be sure, and they wear protection at all times.
I don't think there is anything immoral about solicitating one of either type, though I'd say it is probably dangerous and stupid to go with the streetwalker, and expensive to go with the WILF.