However you go, take the day and go there. There are pros and cons to each method of transportation. If you don't care about a schedule and having to be somewhere at a certain time, then by all means take the bus. The buses run on a certain schedule and you will go where they have it already set up with the casinos.
If you have the day and are a little curious, take the car and go to Oatman, check out Bullhead City, and the casinos on the Laughlin side. Oatman is worth the ride, it is a quaint (dumpy) little town with the wild burros roaming freely. I love going there and if you really want an adventure take Route 66 toward Kingman and go back to Vegas that way over the new bridge at the Hoover Dam.