The easiest would be to drive yourself, but it would make for a long day--it is about 5 hour drive one way, so you would only have a few hours at the South Rim of the Grand Canyon National Park. You'd be better off to spend one night at the park. The park is ideally suited for self-touring.
Check the Top Questions on the Las Vegas forum for information on tours.
You can do this trip in one day...people do it all the time but a few suggestions first!
Always better to rent/hire a car and drive yourself. It's much cheaper and you are on your time!
You will want to leave Vegas as early as you can...between 6:00 & 7:00am. Maybe pack a small cooler with either breakfast items or lunch items...or both! Bring water as well as any other beverage you wish.
Remember to leave the park before it gets dark. There is an abundance of large free roaming wildlife on the roadways!
Go into www.nps.gov/grca to plan your trip before you get there. This will help!
Go to the Las Vegas forums. There are scores of day trip reports, itineraries, reviews and links to providers.
The best trip is the one where you drive yourselves. Be prepared for a 15 hour day.