Having seen Wendy, she's a raunchy type. Don't expect the GFE with lots of hugging and kissing. She does do it but her positives are the excellent OWO, really good in fact and she will do sex in any position, on the bed, standing, on the sofa..you name it. English is terrible so don't expect a conversation.
I haven't seen Crystal so can't comment on her service but she seems to be the softer GF experience, so it depends what you're looking for, or just book both girls and let us know.