Cacturne:
Pros:
- Has good mixed offensive stats
- Gets a good moveset to abuse these stats
- Has Swords Dance and Sucker Punch, a very good combo
- Water Absorb can help it regain health for it's sash in case hazards are on the field
- Destiny Bond works with it's horrid defenses
Cons:
- Very, very frail
- Almost always needs a Focus Sash to be of any use, which disallows it using boosting items
- Slow and relies on Sucker Punch for most kills, which leads to mind games
Shiftry:
Pros:
- Has a decent attack stat
- Chlorophyll patches up it's speed problem
- Surprisingly enough, gets Defog to clear hazards
Cons:
- Below average physical movepool
- Chlorophyll needs the sun; which increases it's Fire weakness
- Frail
My opinion
Obviously unless we know your team, it is impossible for us to say which is better. However, I would go for Cacturne, mostly because it doesn't have to rely on the sun, and has better offensive stats.
But if you have a Pokemon that can set the sun (Drought or Sunny Day) and have access to Gen 6 move tutors, go for Shiftry.
Hope I helped!
