Well, others hav already responded.
But anyways, just to supplement,
I started with Genesis 8, but now I mostly do Genesis 3.
Why I mostly use Gen3 now
The older have a good stack of known quality items, its flagship morphs are significantly superior to the most of the newer morphs and almost everything comes cheaper. It is more straightforward to get a good result imo.
G3 Versus G8
On the G3 vs. G8, well that's a dead horse if there ever was one.
Was beaten to a pulp some time back on all technical forums.
In short, there is no meaningful difference between G3F and G8F for looks.
G8F has few incremental minor advantages in morphs build in and some expression enhancements, thats about it.
It is an incremental and technical step forward not a revolutionary nor one that changes looks.
Assets
G8F has overtaken G3F in amount of assets here during 2020 and that gap will continue to increase, no doubt about that. But then, most assets can be used directly as is and otherwise conversion is supported. Largely a non issue for most things.
Edit: 4 of the 5 most popular tier 1 girls predate the game
As long as a girl belongs to either the G3 or G8 generations the important thing is not the age of a girl, but whether she is a superior build and over time is kept updated (which they are) to keep her superior build edge
