">I've heard that starting at sunrise going down South Kaibab gives you the best views of the canyon as daylight breaks, but also extends the trip to well after dark. Any thoughts, on descending South Kaibab versus Bright Angel? "
I've done R2R2R via South Kaibab once and most of it again last year. I've done rim-river-rim a dozen times via South Kaibab, North Kaibab and Bright Angel.
Yes, South Kaibab has more expansive views than the Bright Angel. But the Bright Angel going down is totally different than the BA going up, so you you'll see different scenery whichever route you choose. If you are conditioned for a steep trail, the SK is faster. But if aren't doing a ton of steep vertical in your conditioning, you might feel better after the longer BA-NK-BA: The beginning down and the final up aren't as brutal. And there is more water, not only piped water at 1.5 mile and 3 mile, but also several creek crossings - I find that dipping my shirt and bandana in the water can really help cool and refresh me, versus sweating out the same amount of water. In early May you could be dealing with very hot temperatures or a late snowfall. You'll know which before you start, but if it is hot, BA has more water. If it is cold, BA is less windy and has more shelters on the way.
For me, there is an emotionally easier aspect of not relying on the shuttle - I don't get up 30 minutes early to be sure of not missing it and if I wake up a bit early, I just start hiking earlier.
Another persecutive in favor of BA: If there's a short-cut bypassing a switchback, the newbies take it and the experienced hikers don't. They know that 500 to 600 vertical feet per horizontal mile is the most efficient slope to climb or descend a slope. BA is at that slope. SK is steeper.
When I've done SK-NK-SK, I've had a car at the SK trailhead so I could start as early and end as late as I wanted/had to.
Since you're coming up BA, going down BA only adds 2.5 miles, but you get to pick your starting time and relax about maybe missing a bus.