They both have decades of experience providing astounding support. I should know as an edge case that has tried to rip the wings off of both their programs (trying to migrate away from disastrous RAID software and ending up with duplicate UUIDs, the sort of thing that never happens to anyone else). This matters even if you never need support: Their software is battle-tested by decades of user experience that they understand.
This leads to software convergence, which this rewrite furthers. At first SuperDuper! appeared to me to be oversimplified to avoid trouble at all costs, while Carbon Copy Cloner appeared to be a collection of enthusiast scripts. One could in fact find the scripts driving either; I expanded SuperDuper!'s scope by directly calling an internal script for years.
Decades ago I used Retrospect. They were arrogant twits but they supported external tape drives and such. My breaking point came when when my life turned bicoastal, and their time zone bug forced full backups after every flight. No MBA stereotype could prepare me for the stupid brushoff I heard at a MacWorld show over this. External drives had become affordable enough to motivate a switch to SuperDuper!
More recently there was a macOS release that put the category into crisis, and Carbon Copy Cloner figured it out first. I switched and I haven't looked back. I loved SuperDuper! and I love Carbon Copy Cloner. How much control surface do you want? You have to see for yourself.