Your format isn't specified, but neither is the best move you can use. Knock Off is better than both, because it gets a damage boost when removing an opponent's item (and most competitive Pokemon have items). Generally, you'll always want to use Knock Off over Throat Chop or Night Slash, unless there's some weird niche case for using either of the latter.
If you absolutely must pick between the two though, then use Night Slash. Neither is great, but Night Slash's niche is a little better than Throat Chop's niche.