I started out using only EOs. I've just branched out to FOs and went a little crazy (this is very common, I hear).
Basically my standard is this: If there's a reasonably priced EO that doesn't come from an endangered species, I buy it. So I don't see any reason to buy a lavender or patchouli or orange FO when I can get an EO for a comparable price.
Rose, geranium, chamomile are hideously expensive as EOs. So I do without.
There are also some scents that you can't get "naturally." Fruits like pineapple and watermelon, coconut, coffee and chocolate (yes there are absolutes, but very expensive), vanilla, etc. If I want those fragrances, I opt for the FO.
Some EOs like wintergreen and almond in their natural forms are highly toxic. So artificial would be safer in those instances. And then there's fragrances like cannabis - which probably wouldn't survive the lye monster and would also be illegal..... :shock:
So now I use both!