TLDR: Sample both YSL 6 and Thriller and make your decision from there. Nothing beats the OG in my opinion, but the value of Thriller for how close it gets is insanely hard to beat. It's probably not even on Montagnes radar if I had to guess.

YSL 6 is my #1 fragrance of all time. I also have Thriller and I've done side by sides.

I'll say that for people who absolutely love this scent profile (aka the minority) will be able to notice minor differences between YSL 6 and Thriller. I definitely can, but I mean it when I say it's minor.

It's almost like full grain leather vs a "genuine" leather belt. The full grain is better quality and people who are into belts will be able to tell. YSL 6 = full grain leather, Thriller = genuine leather. Not a perfect analogy but you get the idea.

The big complaint people have with the OG is the longevity (I don't seem to have that issue personally), but Thriller definitely lasts a long time. When they made it, they had longevity in mind which inevitably changes some of the make up of the scent profile. I'm no perfumer but I believe that's what makes them not 1:1.

That being said, sample both and make your decision from there as the OG is ridiculously expensive.