If I'm not mistaken, the only 520 W unit NOX makes is the NXS520. Apparently its built using components by Channel Well which is generally well-regarded. Its capable of 480 W on its +12 V rail. I can't find any reviews, but it sounds like its probably fine. Maybe someone else can chime in with more info.
If you are trying to decide between two, go with the XFX unit. But I suspect the NOX 520 W is okay.