Go for Payback.
The only gym Pokemon that are slower then Scrafty are:
Milo's Gossifleur
Bea's Pangoro (Tie)
Bea's Machamp
Opal's Mawile
Gordie's Shuckle
Gordie's Coalossal
Pier's Scrafty
Rihan's Gigalith
And that's it! (I did also count Leon's team, but all of them were faster)
Only 8 Pokemon out of the 28 total Leauge Pokemon are slower, and using the moveset that you sent, 6 of them can be hit with super-effective damage. (Machamp, and Mawile are the 2 you can't hit)
Crunch's defense drop is good, but Payback is more worth it because of 100 base power dark STAB (Which turns it into a 150 base power move when the 1.5x is applied) .
So yeah, Payback's better for Scrafty