Nvidia supplies the chip and the reference design to the card maker patnr=ers and they in turn put together thier cards. So when you have the card maker offering a referance card then it is most likely the same as everyone elses and what makes it different would be the name and the cooler that is attached. Some will opt for the single fan and others have a dual fan so some referance cards might cost a bit more.
The two choices that you have , PNY and Gainward are two companies that have been making cards for quite some time and thier reference cards are just as good as any. As far as which would be better is a tossup since we're talking about reference cards and not pumped up cards. They will follow Nvidia's spec's for the reference design so the cards should be about the same. You don't hear much about non-rference cards from PNY and you do with Gainward more so I would say that possibly you could go with Gainward if you were looking for a reason to choose one over the other.