NEC PA322UHD is waaaay bigger than the iMac, so in practice you can get better use of higher res - you can pack more apps and tabs and still have readable UI elements. On iMac you'll end up using HiDPI modes simulating lower resolutions, so the image will be just sharper, there will be more detail on images, but it won't give you better work efficiency than a standard res 27".
Quality wise PA322UHD is the best, most impressive LCD I've ever saw - very uniform, with superb color stability, widest gamut, excellent linearity, calibration and managing software (SpectraView II and MultiProfiler) and all imaginable features for color critical applications.
It's also very robust so it can handle many years of intensive work (even 24/7) - virtually all iMacs sooner or later suffered panel delimitation issues - putting a computer and a display into a thin, aluminium can is not the smartest idea.
30" vs 32"