I think bestworld summed it up pretty well
here. Basically for your two pets...
Have something with the speed boost for crossing mid or going from base to base in AB or anytime you need speed. Bestworld points out mechanicals can't be sapped/polymorphed, so they might be a bit better than any other pet, but really you mostly care about the speed buff.
If you have a pre-BfA carrion bird that still has mortal wounds, thats one pet. Use it when taking down an enemy flag carrier or focusing an enemy healer and theres no other source of anti-healing. You can also use it when playing solo with no healers to heal a bit with the leech.
If you don't have a pre-BfA carrion bird, then basically you get a choice between 10% leech and the 5% HP / dmg reduction active ability. I'd generally lean towards the leech for solo/pug play, but if you find yourself FCing for some reason - then you can take the HP.
In a premade mid scenario (and in many mid fights), you'll want to have no pet for lone wolf's 10% damage boost.