OP since you are comparing a glidecam and shoulder rig, you should be warned that they work in very different ways. maybe these pros and cons will help make the differences more clear
-Heavy. Requires a lot of concentration and is difficult to use for long periods of time. Also difficult to point in multiple direction quickly because of how it works.
-Intended for smooth shots where you have your path planned out ahead of time. Ideally you won't be pointing the camera in many different directions.
-Not nearly as smooth as a Glidecam, but much easier to use for long periods of time.
-Very easy to walk around with and is much more versatile as a stabilizer. However, your shots won't be nearly as smooth.
Devices like the Glidecam were designed for very specific shots in films such as following a character walking. Outside of skiing (or other action sports), it's highly unusual to see people using a glidecam for nearly every shot. While a glidecam could be used for a few super smooth shots in a film, you could use a shoulder rig to shoot an entire film, especially for documentary work. With a Glidecam this would be totally impractical.
TL;DR - If super smooth skiing shots are what you're looking for, get a Glidecam. If you want a versatile stabilization device that won't be quite as smooth but will be useful in many situations outside of skiing, get a shoulder rig.