SFM is much better for animation without question. Ease of use for posing will depend on preference, but I found it much each to pose and set up scenes in SFM as well. Generally, SFM has more compatible models too. Just about every model for gmod will work with SFM, but not all SFM models will work with gmod.
Another important factor for me is stability. SFM has rarely ever crashed for me, and I've never had any issues with updates. Gmod was a mess of crashes and random errors. Updates frequently rendered the game unplayable.
Honestly, the only real pro I can think of for gmod is that it has a whole game behind. There's lots of other shit you can do and posing is just one of them. If you're serious about posing above all else though, go for SFM. Watch Valve's SFM tutorials and look around for some of the great tutorials on this forum and you'll be set.