This is discussed in past threads, so I know there are leather connoisseurs who swear they perceive a big difference. But honestly, coming off twenty years and five cars (Porsche and Ferrari) with full leather -- and knowing how it looks/feels after 5-10 years -- I'm not missing it.
One of my longtime passions (besides European sportcars) is the finest, softest lambskin leather jackets. So it's not like I'm insensitive to the appeal of highest quality leather. But the leather in cars has never been of that quality, while the synthetics have gotten much better -- something I really noticed in my wife's 2016 Mercedes, where I kept running my hands over the textured finish every time I rode in the car, it was that impressive.
My standard S cabin actually has a lot of leather (steering wheel, shifter knob, boot, center console lid, door pocket lids, plus some part of the outer seat according to the literature) and the fact of the matter is I can't quite figure out exactly what is leather/synthetic on the seat (I've actually tried smelling it!). The cabin just exudes a holistic feeling of tasteful sporting purposefulness, without regard to the origins of materials.
Now having said all this, I am such a leather nut and vintage car lover that if configuring a new car (I took mine as-is from dealer inventory) I would still consider leather -- but more likely the premium-priced Natural Leather in Espresso/Cognac, also on all trim. That's like $5k of leather that adds not a whit to the performance envelope...
So if going black interior, and building on a budget (who isn't?) with emphasis on sport driving over luxury touring, I'd be hard-pressed to justify the leather based on my experience. But then I'd probably get it anyway.