Not to get too much into the legalities and etc... but as an activist for sex workers (and clients!) it's important to be mindful of one's language.

Using the word "girl" conjures up just that:  someone under legal age, whatever that may be in the city/state/country where the term is used.  Usually someone under the age of 18.

Using the term "girl" CAN get a person in trouble legally.  Just something to keep in mind.

As far as woman or lady, I've been called both.  I honestly prefer "woman."  Think of Billy Joel's "Always A Woman" song...

"She takes care of herself
She can wait if she wants
She's ahead of her time

And she never gives out
And she never gives in  
She just changes her mind...

She'll bring out the best  
And the worst you can be
...'cus she's always a woman to me