Hello,
I have been calculating some data regarding these movements. Here I bring you my conclusions ( also, feel free to correct me )
Stone Edge
Damage: 100
Accuracy: 80%
Extra: 12.5% crit rate
Rock Slide
Damage: 75
Accuracy: 90%
Extra: 30% flinch
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So starting with Stone Edge, let's imagine we use this move 100 times. Over this 100 times, 20 will fail for the accuracy, so we hit 80 times. And over this 80 times, 10 result in a critical ( 12.5% ):
70 x dmg + 10 x 2dmg = 7000 + 2000 = 9000 damage total.
And now, Rock Slide. Again, we use this move 100 times. Over this 100 times, 10 will fail for the accuracy, so we hit 90 times. And over this 90 times, 27 result in a flinch ( 30% ) that guarantees us to hit again for free, so, double damage, but over this 27, again 3 will fail for the 90% accuracy, and also the resting 24 will result in a 6 times flinching, but only 5 will hit etc... but let's stop at the first flinch:
63 x dmg + 24 x 2dmg = 8325 damage total ( stopping at the first flinch ).
So Stone Edge seems to be more powerful, but we can't calculate the damage we AVOID every time we make the enemy flinch. Maybe we hit with SE, the enemy survives and he kills us, but if we hit with rock side and the enemy flinches, we can hit again, we survive and he dies, etc.
So for me, Rock Slide is slightly better just for this reason.
EDIT1: I jut realized Rock slide can also crit, having a 6.75% chance, so... I'm pretty sure this puts Rock Slide far from Stone edge now.