It should also be noted that Shiftry actually hits harder than Cacturne on both the physical and special side, can with its mixed set alone.What makes Shiftry better than Cacturne? I understand the speed and Knock Off is nice, but Cacturne hits harder and has a Water immunity, and I don't think Defog is that huge of a move on Shiftry.
252+ Atk Life Orb Cacturne Sucker Punch vs. 0 HP / 0 Def Spinda: 259-305 (99.2 - 116.8%) -- 93.8% chance to OHKO
252+ Atk Life Orb Shiftry Knock Off (97.5 BP) vs. 0 HP / 0 Def Spinda: 286-337 (109.5 - 129.1%) -- guaranteed OHKO
252+ SpA Life Orb Cacturne Giga Drain vs. 0 HP / 0 SpD Spinda: 242-286 (92.7 - 109.5%) -- 56.3% chance to OHKO
4 SpA Life Orb Shiftry Leaf Storm vs. 0 HP / 0 SpD Spinda: 253-298 (96.9 - 114.1%) -- 81.3% chance to OHKO
It's pretty clear that Cacturne is nowhere near as efficient as Shiftry at dishing out damage, especially considering Shiftry's better Speed tier; all Cacturne really has over Shiftry offensively is a stronger Sucker Punch, but Shiftry's Sucker Punch is still the second strongest priority in the whole tier (252+ Atk Life Orb Shiftry Sucker Punch vs. 0 HP / 0 Def Spinda: 235-278 (90 - 106.5%) -- 43.8% chance to OHKO). Cacturne does offer some defensive niches over Shiftry in the form of Water Absorb and Spikes, as well as some disruptive moves such as Destiny Bond and Spiky Shield, but Shiftry still has its own bad of tricks in the form of Defog, Early Bird (to tango with Jynx) / Chlorophyll (to fit on / combat the increasingly popular Sun teams), and even Explosion.
Cacturne is by no means bad, but Shiftry has overall more to offer.