Before you read any more, let me TLDR this first: There is no appreciable damage difference between pet attack speeds because damage is normalized. For the most part, it simply adds flavor to different pets.
First, a little history. Back in Vanilla WoW, several pets had different attack speeds (you can see the original list here). The most notorious pet was Broken Tooth, a rare cat in the Badlands, because he was one of only two pets with a 1.0 attack speed (2.0 is the standard). This made him a pretty nice pet to have in PVP because of spell pushback. Mages weren’t getting any casts off with Broken Tooth clawing away at them (in today’s game, pets cannot do this anymore).
Eventually, Blizzard normalized all pet attack speeds to 2.0, and it’s stayed like that all the way from The Burning Crusade… until now.