We contacted allgrandcanyontours.com (operated by look tours or grand canyon west) for Hoover dam and it was a very good experience. However, it's little pricey compared to other operators and we had to book on the same day which cost us 20$ more per head. So make sure you do plan at least a day ahead since on holidays the tours don't travel.
Based on my experience with Hoover dam, it's just a deeper dam but not wider and no active water flow system that you see in your locale. If you're short on time in Las Vegas, then you can comfortably avoid Hoover dam.