For art, I would definitely say SFM.
In my opinion, it just looks a lot cleaner than Garry's Mod.
For animation, I would also say SFM, UNLESS you are doing one of those silly animations that are usually made in Garry's Mod.
I also think SFM might be easier to pose (I haven't used Garry's Mod for art before, so I'm not entirely sure, but having the two views seems much easier).