I think a better solution would be to create a bootable cloned backup on an external drive before you upgrade. Use either CarbonCopyCloner or SuperDuper.
Why do this?
Because if you upgrade and DON'T like the results, having a cloned backup makes it trivially easy to "get back to where you once belonged".
Just connect the clone, boot from it, reinitialize the internal drive, and "re-clone" your backup BACK TO the internal drive. Then you're ready to go as you were before.
An alternative course of action:
If you have a spare external, install a clean copy of El Capitan onto it.
At the appropriate moment, the installer will as if you want to migrate data from an existing drive.
Direct the installer to your internal, and let it import apps, accounts and data to the new install.
Now you can boot and run El Capitan -externally- for a few days, to "get a feel for it", BEFORE you do anything to your existing installation.
Doing either of the above gives you "an easy way back", if things don't go as expected.