The only real problem I have with this movie compared to REC. (spoilers)
The disease itself and the ending of the movie. In REC, moreso in REC 2, the disease is revealed to be demonic possession but in Quarantine it's just an accelerated strain of rabies. The girl at the end of REC is revealed to be the Medeiros girl who is supposedly the source of the possession, but then in Quarantine, there's absolutely no explanation of who that person is in the penthouse at the end of the movie. There's rats in cages and newspapers talking about some sort of nuclear weapon and we're just left to assume that the rabies is a biological weapon they were developing there and someone in the penthouse was experimenting on the rats. And that's not even explicitly explained in the movie, it's just what we're supposed to assume, if I'm right.
Long story short, since they chose not to go down the possession route in Quarantine, the ending doesn't really make much sense to me. And I'm thinking the ending still wouldn't make sense to me even if I hadn't already seen REC before watching Quarantine.
Overall, I think both movies are fine in their own rights, but I will always prefer REC and REC 2 over Quarantine. I'm definitely looking forward to the next two REC movies that are planned.
Don't like me? F_ck off. Problem solved.