Those two are pretty different Pokemon. Despite being both Grass types.
Pros: Great Speed, Good move pool, high S.Attack, Decent Attack, and Good Abilities.
Cons: Low Defenses, slightly limited special move pool, many weaknesses.
Best Role: Sweeper
Best Ability: Unburden
Good Physical moves: Acrobatics(Flying-gem+Unburden), Leaf-blade, Bullet-seed. Swords-dance, Earthquake, Rock-slide, Drain-punch, Thunder-punch, Crunch, X-Scissor.
Good Special moves: Leaf-storm, Giga-drain, Energy-ball, Focus-blast, Hidden-power, Dragon-pulse.
Pros: High HP, High Attack, Great Defense, Great S.Attack, Good abilities, and a fitting move pool.
Cons: Very low S.Defense and Speed, Many weaknesses, and a limited move pool.
Best Role: Tank
Best Ability: Regenerator(Other abilities require sun support)
Good Physical moves: Power-whip, Seed-bomb, Earthquake, Brick-break, Rock-slide, Swords-dance, Growth.
Good Special moves: Focus-blast, Giga-drain, Sludge-bomb, Hidden-power, Growth.
Good Status moves: Synthesis, Leech-seed, Amnesia, Substitute, Stun-spore, Sleep-spore, Toxic.
Choose Sceptile if you need a sweeper, Choose Tangrowth if you need a tank.