STM and USM are different kinds of focusing motors. Prior to STM, USM was the preferred option because it was fast and quiet (relative to other kinds of motors that are available). However, it isn't fluid which becomes an issue with video. Enter STM which is also quiet (though I'm not sure which is more quiet) and provides a smooth transition as it focuses rather than a jerky back and forth. My understanding is that USM is still preferred for stills, but STM is preferred for video.

As far as image quality, if you have both lenses available and you can't tell the difference between two pictures that you take, does it matter? I'd recommend looking at sites that review these kinds of things, but Roger Cicala and Lensrentals.com recently compared a couple of lenses and noted that when doing the tests at the wide angle it has to be done near minimum focus distance which is not the typical use case for that focal length. So try for yourself and use which you feel more comfortable with.