shoulder mounts are better for short films and documentaries. they allow you to get a semi-stable shot for a long period of time (whereas a glidecam will tire you out) and with shoulder mounts you can turn easily and use tools like a follow focus. with the glidecam you aren't able to change focus or anything if you're using it right.
glidecam is for straightforward follows. it's smoother but less versatile
shoulder mount is for run and gun (having to maybe stand still for a long period of time or move, having to shift focus, etc)