We ended up doing the powerplant tour because the Dam tour was not offered during our visit.
The Dam itself is breathtaking and impressive and certainly worth a visit. As for the tour, meh. Interesting enough but not something I would ever do again and I don't think I'd recommend it. The movie at the beginning was probably the most interesting part and the views from outside are more impressive than what you see on the inside. I would recommend you save your money, go watch a video or two on youtube about the hoover dam.
One legitimate complaint about the experience. I'm native English speaker and could not understand about 50% of the tour guide's talk. He had strange pronunciations, voice inflections and would mumble and slur his words together, particularly at the end of a segment.