I like to try them all, but each has their flaws and advantages.
I started with Gmod, so I'm more familiar with it, with the proper tools it works out good for quick posing.
I also use XNALara, but I find there isn't enough "maps" / "locations" for it. ... actually even Gmod has a limited variety.
SFM so far just crashes to often, I want to learn it, but I don't have enough patience to start it up everytime. I installed to many models I think, some don't work, so I'd probably need to do a clean install. Even Gmod crashes on me at times, to many models.