In terms of personal preference, Tentacruel by far. I just find it to be a better Pokémon. But, in true SL-hakase tradition, I'mma analyze their stats and such.
Octillery
HP: 75
Attack: 105
Defense: 75
Sp. Attack: 105
Sp. Defense: 75
Speed: 45
Tentacruel
HP: 80
Attack: 70
Defense: 65
Sp. Attack: 80
Sp. Defense: 120
Speed: 100
Obviously, just by that, Tentacruel can easily kick Octillery's ass. I think I need go no further. .
When you put it like that, I just HAVE to do this, you know :P
Obviously, Tentacruel is faster no matter what, unless Octillery's got somewhat of a Speed pass.
Octillery w/ max Sp. A + natured, and Life Orb using Psychic vs standard Tentacruel:
339 Atk vs 341 Def & 364 HP (90 Base Power): (46.70% - 54.95%)
The same Octillery w/ max A, + natured, and Life Orb using Return vs standard Tentacruel:
339 Atk vs 216 Def & 364 HP (102 Base Power): (40.93% - 48.35%)
(too lazy to run Charge Beam calcs)
Tenta vs Octillery max Sp.A + natured, Life Orb, Sludge Bomb:
284 Atk vs 186 Def & 292 HP (90 Base Power): (66.10% - 78.08%)
However, that is rarely the case, as Tenta usually don't invest in the offensive arenas, nor do it care much for orbs, so this would be more likely:
196 Atk vs 186 Def & 292 HP (90 Base Power): (34.93% - 41.44%)
Obviously, the last one is the most likely scenario.. and so, it turns out to be:
Tenta used Sludge Bomb!
Octillery lost 40% of its health.
Octillery used Psychic!
(It's super effective!)
Tenta lost 50% of its health.
Octillery lost 10% HP using its Life Orb!
Tenta restored 6% HP using its Leftovers.
Tenta 56% HP, Octillery 50% HP.
Tenta used Sludge Bomb!
Octillery lost 35% of its health.
Octillery used Psychic!
(It's super effective!)
Tenta lost 55% of its health.
Octillery lost 10% HP using its Life Orb!
Tenta restored 6% HP using its Leftovers.
Tenta 7% HP, Octillery 5% HP.
Tenta used Sludge Bomb!
Octillery lost 38% of its health.
Octillery fainted!