Grass Knot works on opponents that are heavier than you. Energy Ball deals damage and has a 10% chance to lower the enemy's Special Defense.
As Z-Moves, Grass Knot and Energy Ball become Bloom Doom with 160 and 175 power respectively.
Sceptile is a speedy special attacker, which means that it can utilize very powerful Grass-type moves, like Energy Ball. So, I would use Energy Ball due to it having more power than Grass Knot.