They're just differents, no need in trying to show one outclasses the other. Tentacruel has the useful Bug/Fight resist while having to deal with the Ground weakness (don't forget it gives it problem against SRers like Hippowdon and Donphan), the Poison typing and the Toxic Spikes. Blastoise has better defensive stats, access to phazing, can spin against the one I said, has Foresight and is unlike Tentacruel a good answer to Landorus. I won't even mention the not-ever-mentioned Aqua Jet who can be useful due to the omnipresence of Terrakion/Landorus/Volcarona in the metagame and the fact that Blastoise will be used in rain teams most of the time.