Sceptile has higher Special Attack and Speed, while Decidueye's other four stats are better than Sceptile's. Neither Pokemon is amazing competitively, but they have their niches. Sceptile is a fast special attacker with interesting options like Dragon Pulse and Focus Blast. Meanwhile, Decidueye is a slower attacker that can go physical, special, or mixed thanks to Swords Dance, Nasty Plot, and powerful moves on both sides of the spectrum.
The cliche answer is that if you're just playing for fun, it only depends on your personal preference. Both Pokemon will get you through a playthrough just fine. In terms of competitive play, I'd recommend looking up the viability rankings for whatever tier you want to play and see which of the two is ranked higher.
