Once you are verified with these services, they will give you a code to give to women, who can then look you up. (Each is slightly different, but it is the same concept.) The whole point of this is to combine references from ladies, with one employment screening by a third party - and have a lady you would like to see be able to check that in one place instantly.
I happen to like Preferred 411 the best, but that's just a personal preference. It's free to ladies indefinitely, and I like the interface better. Which one makes the most sense to you will depend on where you are located. Try taking a look at women's site's nearby or posting on a local board to ask which is most commonly used.