The Pokemon depends on your team.
Scolipede is a great sweeper with great attack and speed, good for Mega Horning, Poison Jabbing and Double-Edging the other team, also being the world's second best speed Baton Passer.
Crustle is a physically defensive wall, with nice attack for using the most powerful Rock move in the world, 150 BP Rock Wrecker (A huge 225 BP with STAB included).
Use Scolipede if:
- You have 0-1 Physical sweepers. (Also applies for Crustle)
- You have 0-3 Physical/Special sweepers.
- You need quite a strong Grass coverage.
- You want to slap Fairies out your path.
Use Crustle if:
- You have 0-2 Bulky walls.
- You have no physical walls.
- You want to have Ice, Flying, Bug or Fire coverage.
- You don't find 'risking losing a turn' risky/You use Slaking a lot.
Also, if you are asking just to find out if you want to use a mono-Rock/Poison team with a Bug, I advise using Crustle as Rock Pokemon have lots of coverages and resistances (despite being scared of water) and are immensly powerful.
If you want some sets for the two, here are some nice sets.
Scolipede @ Focus Sash
Ability: Speed Boost
EVs: 4 HP / 252 Atk / 252 Spe
- Swords Dance
- Poison Jab
Crustle @ Choice Band
EVs: 248 HP / 252 Atk / 8 SpD
- Rock Wrecker
- Bug Bite/X-Scissor
- Knock Off
- Shadow Claw
Crustle @ Leftovers
Ability: Shell Armor
EVs: 252 HP / 252 Def / 4 SpD
- Stealth Rock
- Smack Down