The difference is in the type of motor used to focus the lens. The focus motor in STM lenses are more or less silent to the camera's audio system. This is important when shooting video but not for still photography. This may help.
In addition to being "Silent", there are other important advantages to a "Stills" photographer with the STM version.
The STM AF motor changes the design of the entire AF assembly so it is now "Internal Focus" This means that the front element of the lens does not rotate or extend during focus. The focus ring also does not spin by itself during Auto Focus, and you can also make manual adjustments after focus has been achieved.
The STM version has also been reported to be sharper than the non-STM version.
These are all very good reasons to consider paying extra for the STM version.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.