I bought the NCT hypnobirthing CD, and , to be honest, it annoyed the hell out of me. The woman's voice was not at all hypnotic, just boring, and there were a couple of times when she said something vague, like "Think of a beach", and I did, and then she went blathering on about some specific things about the beach which contradicted my view of the beach! The "script" didn't make nearly enough use of effective words such as "strong" and "powerful" and "open", but blathered (again) about "wonderful" this and that, which I found incredibly weak.
As you can tell, I lost respect for it quite fast!
I think a class would probably be better, as it will be interactive, and not dictate what your birth visualisation is.
And please make sure your visualisation does not break down over interventions. This must be the major psychological danger area, as not being prepared for "things to go wrong" is just wrong. If you wanted to be in control in the first place, by taking a hypnobirthing course, the idea that things could go "so badly out of control" would be even more frightening than for a woman who is aware of all the options.
I hope I have not put you off one or another method (except that CD), and that this has been a fair post! Good luck.