The shader system is a bit better with the skin. The eyes sometimes are better looking. G9 facial expressions are more convincing sometimes. The material maps for the body are completely different. And probably the biggest thing is that that poses for G8 don't work on G9. So you have to buy either a pose converter for your G8 stuff to work on G9 or buy all new pose packs. A lot of the clothes and hair don't work between G8 and G9.

Close up portrait work seems to be better with G9.

Honestly it all comes down to personal preference. Some people immediately jumped to G9 and never work with G8 anymore. Some people refuse to work with G9 because they have so much still invested with hair and poses and clothes and stuff with G8. Some people think G8 has a lot of life still in it. And some people believe that G9 has a lot of problems and was a rushed to market as a money grab by the Daz people.