no one seems to have noticed a huge difference in pets. My level 18 moa bird usually outlives all stalkers that i've partied with, but a lot of people seem to believe the bear is the best. personal preference, methinks.
I have a wolf lvl 16 and the other guy had a bear lvl 16 and i watched them attack and mine did more damage because i put more skill points into my animal all about the skill points not about the animal