A year ago today I was heading down 240 miles of the Grand Canyon. Its beauty and immensity are staggering. Which rim you'll be on will determine what you'll be able to see, but in general, the canyon is so big it doesn't matter where you are as long as you can see into it.
For optimal viewing, if you can find your way to the Hualapi Sky Walk, make it happen. It's 4,000 feet above the Colorado river on the south side of the canyon, opposite the "Bat Cave" (the bat cave is off limits to tourists now unfortunately). Anyway, the sky walk literally loops out over the south rim so you can see straign down. Very cool.