Yes, Cloyster is better
This is because Shell Smash effectively doubles attack, special attack, and speed, and your guaranteed 5 hits would certainly can break Substitutes and Focus Sash that Keldeo can't. It also has better coverage than Keldeo (though with Hydro Pump rather than Spike Cannon).
While Keldeo is better on the first turn, post Shell Smash Cloyster is an excellent late game sweeper. Although, I'd suggest you run Hydro Pump rather than Spike Cannon, as it lets you deal with Steel types more effectively.
And if you can get one, a King's Rock is also an excellent item as every hit has a chance to make your opponent flinch, so if you're sure that you can survive 1 hit, that would make it even more potent.