Every pet has advantages and disadvantages for everything. I myself have a 45 Hunter on a PvP server and use Humar (lvl 23 rare cat from barrens) and am BM specced. I have had absolutley no problem whatsoever with him, I love the way he looks (reason i got him) and he's a cat which makes him have good DPS.
The fact that i'm BM specced gives my cat and I higher armor and health ratings than a different build, plus the fact that I have 250+ agility gives us both higher armor ratings and higher crit rates which makes my Go for the Throat ability a cloth/leather wearer killing machine. I grind to lvl almost exclusively and don't have any problems taking on 3-4 same lvl mobs with my cat tanking 3, me kiting one and doing it one by one; it can take a while but I don't die from it normally, only if my cat and i are not at full health and Intimidate is on cooldown.
I LOVE my build, in BGs i can easily (given a one on one fight) take on any lvl 49 cloth wearer and win, not all the time as different skill lvls and abilities and all kinds of other stuff factor into it, but I don't ever see a lvl 49 in BG and go oh crap im gonna die, I say sweet gonna get 4+ honor for this guy!
Pick the pet you want to use and make it work for you, no pet is better or worse than any other, just depends on how you play.