1) Quite a bit.
UberEnvironment is an IBL lighting solution based on an enviroment dome that supports HDR tifs and adds light-based ambient occlusion. It’s free/built-in to DS.
UberSoft is an IBL-like lighting solution based on individual types of lights, like distant lights, area lights, spots lights, etc. Most importantly UberSoft uses a single shadow pass for all lights. This cuts down on rendering times substantially.
2) UberSpots/UberPoints are the original IBL spot/point solution for DAZ Studio. They are build at the RSL level (renderman shader language), support gels and falloff.
Muusa Ethereal Lighting uses ShaderMixer built lights, don’t support falloff (to my knowledge), but are otherwise quite similar (except come with different gels). Once bonus is that these contain a distant light gel solution (you have to have UberSoft to get that from the omnifreaker products).
3) I own all the Uber* products. Each has it’s own uses. I didn’t buy into the Muusa’s because I already owned all the Uber products.