The Quest 3 has continuous adjustments, whereas the 3S has three fixed positions. Which are designed to accommodate the majority of head sizes. The middle settings work perfectly for me.
Also, the Pancake lenses used on Quest 3 help reduce light flares and chromatic aberrations. And can accommodate higher resolutions a little bit better.
And on the topic of resolution. This is the most significant difference you need to be aware of and another reason why the 3S is more affordable. The Quest 3S’s resolution maxes out close to 4K resolution of 1832 x 1920, with a respectable 773PPi. For context, most flagship smartphones today have around 400 to 450 ppi. And VR headsets must be higher because the displays are close to your face.
The Quest 3’s resolution is significantly higher at 2,064 x 2,208, with a 1218PPi. So, the image will look sharper, and you will have an extra 5 pixels per degree in the virtual space. But if you’re not intending to play graphically intense games. Then you won’t notice a huge difference. Now audio.