Kind of strange choices.
The iMac is a better computer with or without a screen.
NEC makes multiple models with the PA series (perhaps there is a newer series) that are meant to be used for graphics and thus intended for proper calibration from the get go. You didn't say which model so no way to know if its worth the price.
Obviously if its the graphics version and you have the bucks, get both the iMac and the NEC.
I use a Mac Mini quad 2.0 with a 24" NEC PA24IW and with the 16 gigs of RAM, it works well for photo work. Your mileage may vary.