I know this is a very late response, but I also know you're on here a lot 🙂
Genesis 8 is the way to go IMO. It doesn't achieve the look of some of the G3F models that make me swoon, like Nancy for G3F, but a lot of the characters are awesome and it's fun to create new ones with the skins and sliders. I've only ever used G8F and the only reason I would use G9F is for a few of Addy's models (woah).
The main problem with G8F is the figure's bottom lip as well as the challenge of tinkering with facial expressions to make them less dead or contrived. I feel like the bottom lip problem is the main drawback. I mix in Amira smile morphs with flirt or one of Z's pleasure morphs to get my best version of a genuine and/or sensual smile.
The lips tend to be huge, and when you think they look pretty good with a neutral expression, as soon as she smiles the lower lip looks like a sausage, and it's hard to fix it with morphs. I've used lip depth, lip lower suck, lip lower in/out, etc, with limited success.