I would suggest against Sporebats in PVP. What they do is put a debuff on the ground in a cloud, which works great for slowing down groups of enemies, but can be moved out of. If the caster realizes that it's a debuff zone, all they have to do is move out of it. Not saying they *will*, but it's more likely.
Core Hounds have the advantage of Heroism, if you're in a world or battlergounds PVP, though it doesn't work in arena. If you aren't Beast Master, the cast-slowing cloud of the Sporebat is your only option for 'slowing casting speed'.
For interrupting pets, Gorilla has the shortest cooldown of its interrupt, while Nether Ray and Moth both can do it at range, though I don't feel that the range is as useful as the quicker CD, IMO. Also, unless they've been hotfixed again, none of the pets cast 'intelligently'. I.E. you will have to manually tell them to interrupt, so have a keybinding ready.