Well, Hexagon and Blender have the big advantage of being free, I think Hex is much easier to use, and there is a forum here as well for it. 3D Coat has a fully working 30 day trial, don’t know about ZBrush. I forgot about Sculptris as well, also free which may work for some kinds of morphs too.
I would use Hex becuase I am used to it anyway, plus the bridge as mentioned before. You don’t really need to know how to model in Hex, jsut pull vertices around, and use the ‘Soft Selection’ option when you select a vert, so that others in the vicinity move as well. The paint type programs should be easier I expect, like 3D Coat and ZBrush, but maybe not. Any of these programs have a bit of a learning curve.