In all, they going to have to do a lot of research what they want. ASL is the language that the deaf speak, but it lacks the grammar that the hearing world writes and speaks in. Last I was told, there are 9 different sign languages in the states alone(which is one thing I have to explain to everyone I work with about other countries as well). I know ASL, English sign and CUED are the highest on the list. It mostly depends what kind of school they want to send their child too. My wife was upset cause she wanted to go to a public school and learn English sign(her grammar was bad when I met her). My best friend loved the public school, but had to learn ASL before going to college, but she learned grammar before going to college.
In reality, it comes down to how the child can learn the best with. If he a fast learner, then one way might be better then the other. If he takes a while to learn, then another way might be better. It not something that we can go and say, "This is the way to go" or "your better off doing this." It more long the lines, how fast does your child take in info and learn. It best to learn English sign for hearing ppl, cause the grammar in signing is a lot different in ASL then in English Sign. Also, ASL easier to learn as well cause it short to the point, but it is not signed the same way we talk.