"JoePublic- Is the improved bending due to the superiority of Genesis, or weight mapping ?"

That's not easy to answer.

Weightmapping by itself does not much to improve bending.

Lets say a really good rigger can rig a mesh the old fashioned way until the joints are 80% perfect.

Then the same rigger could bring the joints to perhaps 90% perfection with weightmapping.

Still, to get a joint perfect, you need JCMs. And to create JCMs, you need reverse deformations.

Both Studio's CCT as well as ColorCurvators MorphLoader can do reverse deformations.

Now the thing is, reverse deformation can bring both conventional as well as weightmapped joints to a full 100% perfection.

So, weightmapping by itself isn't actually such a big deal because if you knew how, you could have figures with truly perfect joints for years.

Reverse deformation tools are.

 

Back to Genesis. It bends so well because of careful rigging. Weightmapping sure helped. But I have re-rigged meshes bending just as well without weightmapping.

The big deal about Genesis is the scaleability. This allows you to create wildly differently proportioned figures on the fly without the need to painstakingly re-rig them.

The rigging is great. But other figures have great rigging, too.

Other figures even scale pretty well in Poser. (Though it's tedious to set that up properly)

But Genesis has great morphs and rigging and scalebility.And all work with each other without one feature compromising the other.

And that, so far, makes it superior.