I would recommend looking at ads for the ladies in whatever area you might do some hobbying, and see what services get mentioned.  I used to travel to Dallas on business fairly often, so Preferred411 was the obvious choice, given how popular it is in that city.  Someplace like Chicago might mean that RoomService2K would be a better option.  No service is going to cover everywhere equally well, but DateCheck, P411, and RS2K are three of the biggest.

In Dallas, a P411 membership with provider okays was the next thing to gold.  I had a lady check my references while I was on the phone with her, saw that they were very good, and decided that she could indeed do an outcall to my hotel on very short notice.  I could probably have gotten screened via references, but it probably would have taken longer.  Ideally, you plan things out well enough that you're not looking "on the fly", as it were, but sometimes things fall through.

I have always liked the P411 service, and I've had good experience with the staff.  However, I have never had anything more than a trial membership with any of the others, so I can't give it a fair comparison.