>>19524618Lightningrod is plain more useful than Sap Sipper, especially on a Grass type that's already immune to the Powders/Spore.
Grass/Dragon isn't exactly good but it's not terrible either. It gains weaknesses to Fairy and Dragon but it becomes basically immune to Water, Grass and Electric, keeps its resistance to Ground and loses its weakness to Fire.
Grass/Ground means it loses its resistance to Water and Grass, only gains a resistance to Rock and still has a 4x weakness to Ice - the only direct advantage is losing its Poison weakness and gaining an Electric immunity which it would effectively have anyway even without Lightningrod. Grass/Ground would not be weak to Dragon and Fairy, but is also only resistant to Ground and Rock.
Offensively, Grass/Dragon is better coverage than Grass/Ground.
Weaknesses is not as much of an issue with a very fast Pokémon like Sceptile - it's fair to assume M-Sceptile is going to have at least 150 base Speed, meaning almost no Pokémon are going to be able to outspeed it without Priority shenanigans. Even its base form has 120 Speed, which less than 20 Pokémon outspeed.
A 4x weakness, especially to Ice as a number of other Dragon Pokémon know, is not a crippling setback. Most Ice threats come from Water-type Pokémon which have obvious problems with Sceptile (especially in Doubles with Discharge support), or Mamoswine who OHKOs Sceptile with Ice Shard but gets OHKOd in return by Leaf Storm/Grass Knot/Focus Miss.
TL;DR Dragon > Ground
