I have done both - rented a car and took the bus tour. I would do the hoover dam bus tour, as it isn't all that long, they do pick you up at the hotel, and you get to do the dam tour and relax on the way back. For the grand canyon, a car is the way to go. We went by bus the first time, made for a long day, and after spending 6 hours on a plane to get to vegas, I didn't enjoy spending 8 hours on a bus. We drove there, most valet services at the hotel will gladly hand you a map and tell you the best areas to see. We actually went to the area where the glass walkway is located and did the indian lunch barbeque, it was a terrific day. We took my friend from Canada, she had never seen the canyon. On the drive there, we pulled into a rest area where some Indian women were making and selling jewelry - I spent $25 and got some lovely pieces. We also stopped at some saloon - right out of 1940 - man in the rear strumming on a banjo - he had one tooth, I think - bar with stools that did not match - tables outside made from those wire spool things - it was quite memorable!!!!
Enjoy your trip.