I tried to use Gmod before but I was just profoundly shit at it, even with help.
I just found it difficult even though I used a lot of addons and stuff to try and help myself.
I prefer SFM because if there's a tool or something I can just find it in a few clicks, it's like layers on Photoshop and is mega organised. (Idk if there's smth like this for Gmod).
Though SFM lacks in some ways because there are some maps/models that don't work that I really want to use, or aren't really supported. After a while of using the same map over and over again you have to rely on scene building, but eventually you kind of just run out of stuff to do and make the same scenes over and over again, or at least I found that anyways. Which is really why I stopped.
SFM has limitations to creativity imo but can be a bit easier to use and to create cinematic pictures.
Gmod has far less limitations, and can be much more complicated (Imo) if you're making a complicated scene. But probably after a while you get used to it and adapt.
However I've only ever tried to use GMOD about 4 times to make a pose and gave up.
I've used SFM countless times.
So it's a pretty bias opinion
but just an opinion.
xxx