Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
the iPad Pro is rated at 600 nits. does the wide color (P3) make a difference?

Depends who you ask (and probably what you do). IMO the retina MacBook is fine for my usage and plenty bright (the biggest difference I see is when using it outside or in a very bright room) and the sRGB is fine for me. My mother is now absolutely crazy about the high brightness/P3 displays on the MBP/iPP. I don't do much photo editing...where as that's her primary activity.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.