A lot more people like to visit Las Vegas than Phoenix. It is also often cheaper or more convenient to fly into Las Vegas, and the couple of times I checked, car rentals were cheaper in Las Vegas.

If the above isn't true for you, then Phoenix is a good bet.

A lot of people are doing a circle tour starting with the Grand Canyon and making a circle to Lake Powell, Bryce and Zion and then Las Vegas makes a lot more sense.

Las Vegas can be cheaper to fly into too.

If you are only going to the Grand Canyon then Phoenix, AZ can be a good choice because you can also visit Montezuma Castle National Monument and drive though Sedona, AZ.

Utahtea

Not only that, but a stunning sight can be had when returning to Vegas after dark, upon nearing civilization again via highway #93, and coming over the crest of the hill, and being overwhelmed by the bright lights of the Vegas strip.

We've used Vegas for a couple of reasons: first flights are usually less expensive, second car rental is less expensive as in hundreds of dollars per week, third there are more non stop flights from where we fly.

Either city has interesting things to see along the way and DH finds driving in the west generally easier and more relaxing than in the east. Do a little research and see what works for you given your time constraints and interests.

If you fly into Phoenix, get off the interstate at Hwy 179/89A and drive through Sedona and Oak Creek Canyon on your way to Flagstaff---pretty drive.

Depending on where you are coming from, Allegiant Airlines has good fares to Las Vegas and Phoenix-Mesa. They also have good package deals on tours, shows,hotels and rental cars.
The drawback is that they only fly to many locations two days a week. allegiantairlines.com

Maybe the flights are more expensive to Phoenix from whenever people fly?

For Grand Canyon, we flew to Phoenix and stopped in Sedona on the way.

Out of Las Vegas we went to Zion/Bryce area.