It depends on your modus operandi and priorities once you have your internal HDD fail. Restoring from a TM backup takes twice as long as from a clone, and you have downtime during the restoration. If you restore from a clone, you can boot from the clone and clone the data back onto the repaired HDD or new HDD/SSD and still keep working.
I prefer CCC just for that.
For instance, I have one 500 GB HDD for my photographs (digital and analog) libraries and editing documents, one 500 GB HDD with my personal video footage in an editing friendly format.
Both 500 GB HDDs get backed up to one 1 TB HDD via CarbonCopyCloner.
And that 1 TB HDD gets backed up to another 1 TB HDD via CarbonCopyCloner.
Therefore I have three copies of my important data.
Okay so I'm starting to feel like I'm wasting one of my external hard drives. It's 1 TB and I split it 500 GB to a Time Macine and 500 GB CCC. They both do backups of my 500 GB hard drive and I'm wondering if I need to do both?
....
the cat deciding that everything on the desk needs to be on the floor, etc.
I use both a TM and a full cloned backup - which is what CCC is doing. I use TM to recover from user errors. Files I deleted by accident, versions I overwrote prematurely, etc.
The cloned backup is for full disaster recovery. If the HDD crashes you can simply copy the cloned copy onto a new HDD. While waiting for the new HDD to arrive you can boot from the cloned backup and keep working. If the entire computer is toast, you can borrow a computer and boot from the cloned backup and keep working. However, you can't recover the old files like you can from a TM disk.
However, keeping both backups on one external HDD is not the best practice. One of the disks can perhaps be attached to your router to get it out of the way? In my case I keep my cloned backups next to the computer because I also rotate these into a safety deposit box since the whole point of 'disaster recover' might include recovering from something that affect the house. Most people think about fires, floods, etc and those risks are pretty rare. But you also need to think about the bathtub upstairs overfilling, the bookshelf falling of the wall, the cat deciding that everything on the desk needs to be on the floor, etc. All of these rather mundane and very small "disasters" can take out both your computer and both your backups.
Luck.
Well, that's fine UNTIL you need to restore. All one has to do is browse MacRumors.com over time to see post after post after post after post after post after (had enough?) post from folks to the effect that, "Help, I can't boot, I'm trying to connect my T.M. backup and it won't mount", ect., ect.