It's hard to tell because the NF-12s are new fans and no one, apart from Tom, has really done any tests on them.
The GTs are good fans, and they are know as the best fans for radiators almost everywhere. The Noctuas though, because of their focused flow, like what Tom was saying in the review for them where they blow the air strait and not off to the side like other fans is good for rads.
Ones that blow off to the side and all over the place create dead spots on the air flow, so where as the GTs have a good static pressure to push the air through the rad the Noctuas have a more direct air flow which blows strait through the rad.
So far, I have only seen Toms tests using the H100, whether they would do as good an a full sized rad as they did on the skinny H100 I don't know, but it would be interesting to see some tests done on other rads. Even on full the Noctuas are quiet compared to the GTs running at the same speed which are loud.