We know for a fact that Matthews is the better offensive talent than Eichel, seeing as he has outproduced him at the equivalent age/level for something like 10 years in a row.
Eichel has a 1 year head start, so it is only a matter of when, not if, Matthews will surpass him. Some would say it already happened last year, with Matthews badly outplaying him at the WCoH. Matthews also had a better points-per-60. But Eichel did outpace Matthews in PPG production, which is the simplest metric to look at, not to mention having better linemates.
So I think Matthews wa already the better player last year, but this year he will get more ice time, and that will ultimately be the difference. He won't just be ahead in PP60 next year, he'll also be ahead in PPG.