A zombie game usually goes two ways: Soldier dominant and Zombie dominant. Depends on those two situations and other factors like locations and numbers of soldiers/enemies, your gun would make a great difference. The RPK is obviously good for stopping power and defending, while the Barret provide support damage or straight out zombie murder from a fortified or safe location. So to me, it's not which gun is better, it's more like "did you make the right choice choosing this gun for this round."
my way of thinking "if zombie refers to HM/HMX/MM"
1- an SMG like Thompson Infernal Dragon is perfectly fine for hunting, you got accuarcy, high fire rate, and a clip is enough to kill 1 berserk or 2 slug mutants but the clip size is too small for it's fire rate so i recommend buffing it up by buying 7 VIP thompson which will give you a total 119 bullet per clip and turn it into an absolute killing machine
2- though the thompson is a fine option also it has much faster reload speed it's still quite a bit slow, for all VIP's rifles has the best reload speed, but with very low ammo per clip, M4A1-S-Pretador has it all, special ability it gives you 50-bullet-clip only for HM/HMX/MM, high rate of fire, fast reload speed, higher damage than SMG's, higher jump (helps you escape from very tight situations), melee ready to finish them off, they won't even touch you
3- don't prefer escaping and running, are you a more of a "BANG BANG MOTHERF***ER" type of person? get your self an RPK-Infernal-Dragon and would be sweet if you buy a bulk of 7 only if you want 290 freaking bullet in a single clip
my way of thinking "if zombie refers to challange/AI mode"
dont buy a VIP better save that money and risk it all on an AA-12-Buster this one is a bad mofo