I think Lamb has surpassed Chase a bit for fantasy football, though that might be recency bias.
I'm thinking recent durability, system, quarterback, and coach.
I would personally take Chase over Lamb, but at some point, the points scored talk and talk loudly.
In the end, I'll contradict myself again and say it's a coin flip and I wouldn't blame anybody for choosing the one they do.
One thing is that Chase played traditional X much more than Lamb did before this year, at least to my eyes when I watched. Lamb was a flanker more than anything. Gallup plays or played X in Dallas's offense for the most part. Gallup seems to have lost his job and now Tolbert might play X.
But really, AJ Brown, Chase, and Jefferson are different from ARSB and Lamb in that they have a higher degree of difficulty in their role, which is tethered to the line and stationary and fighting press coverage more often than that. ARSB and Lamb are hidden more.