-There is absolutely no difference between pets strengths
-You should get a pet that most appeals to you... here's a link with all the prophecies pets (pix included)
http://gw.gamewikis.org/wiki/Pets
-Over-rated? No... There are builds that effectively implement the use of pets (Bunny Thumpers, RaO's if you ever decide to get NightFall)
-You do not need a pet.
-Pets are used as another source of damage/melee and/or a 'meat-shield'. Best case scenarior: Your pet takes aggro while you use ranged attacks as a ranger.