These programs have a much different philosophy, though they essentially do the same thing. Clonezilla copies every single active disk block in a partition, while SuperDuper copies most of the files.

I have never used Clonezilla.

I haven't used SuperDuper in a few years, but when I did use it I had no problems. I switched to Carbon Copy Cloner because it had a few more features. However, you need to pay for CCC (there is a free trial). SD is completely free if you just want to clone entire disks, and you only need to pay if you want extra features.

When I say that CCC and SD copy "most" of the files, what I mean is they attempt to be smart. E.g. they omit files in a .Trash directory, and they omit various cache and temporary files.

IMO if your data is important to you, then you should clone it to two separate drives from your Mac's HDD. Which means you can use the free SD and the trial CCC and make one clone with each. Better safe than sorry.

Even at that, whenever I make full backups I like to use an independent method to check that the data is correct. Previously I used to do something like this at the root of both the source and the destination:

   find -x \
        . \
        -type f -print0 \
        | xargs -0 -n 100 -x md5 \
        | sort > /tmp/manifest.source

I would then compare the two result files and see what was copied. Unfortunately that tends to be noisy because as I mentioned not all files are copied. I eventually wound up writing a Python script instead of that shell script. My Python script helps cut down on the noise by e.g. not checksumming files in .Trash directories.

I would much rather trust a program like CCC than Clonezilla. The CCC developer is very responsive to email and to bug reports. His program exists to make backups of Mac partitions. He doesn't need to worry about reiserfs, btrfs, ntfs, etc. Instead he spends his time worrying about quirks in OS X, nee macOS. And that's all. I trust him to get it right because that's his livelihood.