Some are better than others and there are even bargain brands out there. I am currently using 3x 230mm's that came with my Rosewill Thor v2 case and they are silent and my case is like a wind tunnel as it also cools the underside of my desk. I'd look at the different posts around these forums for good 200mm fans. As for 120mm's, I hate them as 95% of them are loud at decent airflow so I try to keep away from them. I have two in my system and they are bolted onto my Hyper212 and they hardly ever hit 1500RPM.
As for what's better, the more air you push through your system the better. But it is better if you have positive case pressure where the fans are bringing in more air than they are pushing out. This helps reduce dust in your system as it keeps from drawing air into every crevice. Also it keeps a positive airflow through the case where as a negative pressure will actually cause small disruptions in the airflow and can actually reduce performance. While it doesn't hurt a lot, it can be noticeable.