There are two major hiking trails from the South Rim of Grand Canyon that lead to the Colorado River and Phantom Ranch: Bright Angel Trail and South Kaibab Trail. Both trails have unique characteristics that may affect your choice.
Although I personally favor Bright Angel Trail, South Kaibab is not without merit. First, the views from South Kaibab Trail are spectacular. Second, South Kaibab Trail is shorter in length, it is only 7 miles (11.3 km) to Phantom Ranch, while Bright Angel Trail takes 9.5 miles (15.3 km, although it is only 8 miles [12.9 km] to the Colorado River at Pipe Creek Beach). And finally, South Kaibab Trail is slightly less traveled than Bright Angel Trail.
However, South Kaibab Trail has drawbacks. Although it is shorter in length than Bright Angel Trail, South Kaibab reaches a higher elevation, 4,860 feet (1,480 m), compared to Bright Angel, 4,460 feet (1,360 m); and the height is reached over a shorter distance, meaning South Kaibab Trail is much steeper. Because of the steepness, the National Park Service recommends only hiking down South Kaibab Trail, then up Bright Angel. This puzzles me. I have spoken with dozens of older hikers who were shocked at the roughness and large step-downs on South Kaibab Trail. These drop-offs take a toll on the knees, ankles and backs of hikers of all ages, but in particular on older hikers. To me, the only way to hike South Kaibab is up (having descended on Bright Angel Trail), not down. Bright Angel Trail in contrast is better groomed than South Kaibab and has more gentle step-downs.
My second concern with South Kaibab Trail is that the trailhead is accessible only by bus. Private cars are not allowed. If you are staying at one of the rim lodges that means you must either catch the early morning “hiker’s express” (about a 40 minute ride), or catch two separate buses to get to the trailhead (30 minutes if you are lucky, or more than 45 minutes on an ordinary day). On a busy day, you might even have to wait in line for a second bus before boarding. To me, the goal is to hike, and if you leave a rim lodge, you can hit Bright Angel Trail in a matter of minutes. I would rather be in motion than sitting on a bus.
Third, South Kaibab Trail is routed so that it is fully exposed to the wind and sun for almost the entire length of the trail. Depending on the season, Bright Angel Trail provides some shelter from both the sun and the wind. Also, South Kaibab has no source of water along the trail. Bright Angel Trail has fresh water at Indian Garden year-round, as well as water at the 1.5 mile and 3 mile rest houses in the warmer months. You can carry a lighter pack on Bright Angel Trail.
Fourth, Bright Angel Trail has a ranger station about half way along the trail at Indian Garden. This is a place you can seek help in case of an emergency. Although South Kaibab Trail has emergency phones, the ranger station provides some security. Further, there is a campground along Bright Angel Trail at Indian Garden. If your hike is going particularly badly, you could spend the night, an option not available on South Kaibab Trail.
Finally, there is the matter of time. On a recent hike I spent the night at Phantom Ranch and chose to hike up South Kaibab Trail. Although the distance on South Kaibab is shorter, it is steeper, so I made slower progress. When I reached the top, I arrived 15 minutes earlier than if I had hiked back up Bright Angel Trail. But then I had to wait for the first bus and then transfer to a second bus, a process that took 45 minutes for the return to Maswik Lodge, where I was staying. All- in-all, including the bus ride, I would have arrived at my hotel 30 minutes earlier if I had taken Bright Angel Trail.
To me, South Kaibab Trail is an interesting alternative route back to the South Rim, best hiked up. Athletic, young people regularly run from the rim to river in 1.5 hours, a trip which takes me 4 hours. If you are one of them, South Kaibab Trail may be great fun. However, if you are making your first hike to the Colorado River or Phantom Ranch or if you are over 50 years old, I strongly recommend you take the smoother, safer, cooler Bright Angel Trail. Good hiking to you.