I prefer mapping for GoldSource currently because I hate the way the source engine over complicates things.
Say I want to make a single player mod based in modern day London.
Right away I'm going to need a prop modeller or 2, a texture artist and everything else that goes along with getting a model in game and working properly.
Hell, even if I want to make a City 17 related mod, just adding custom speech means recording the sound, loading it into FacePoser to do the lip synching and facial expressions and fuck knows what else.
I can create a fairly unique environment in GoldSource with Hammer, Milkshape and my mic in a day or 2.
In Source, it would take a team of 5 about a week to get the same result.
That said, The Core is my last GoldSource project. I really need to get with the times... It won't be easy, but the first few years of GoldSource weren't either.