They are pretty much the same, but GraphicsMagick's big CLI improvement over ImageMagick is that everything's based off an executable called 'gm'. IM's horribly generic 'convert' becomes 'gm convert', etc.

I'm not going to defend their CLIs beyond that though. The fact that 'mogrify' modifies an image file in-place and 'convert' doesn't has bitten me more than once.