I'd say Knock Off. It's important to remove opponent's item(s) to cripple them.
Generally, Sableye has Will-o-Wisp, so opponent attack is halved, meaning Foul Play damage is also halved. In another case, if a Special Sweeper appears, they will mostly have low attack so again, Foul Play is not helpful.
Of course, this is more the general meta (online) games.
In X/Y ingame (based on your tags) either is fine, to be honest, but I suggest Foul Play, because most ingame opponents lack an item. This reduces Knock Off's power by a margin, so Foul Play is generally better.
tl;dr: Knock Off for Online, Foul Play for in-game.
Edit where edit's due: (Damn I love this, I'm using this everywhere)
Foul Play damage is not halved by opponent Burn.
Guess you DO learn something everyday. Too bad I had to make a mistake that involved answering someone else, I feel bad T-T
That said, answer isn't changing, because it's still moderately correct :/
Thanks a bunch, sumwun (and JarJar too) :D