For purely digital photo editing the Retina is amazing. It's possible to see if an image is in focus just by looking at the thumbnails, thumbnail size dependent of course.
If you're referring to colours between the two screens, I believe you're still better off with the Retina. It's still a good idea to calibrate if you're planning on printing but in my experience so far, the rMBP really isn't that bad, some colours are a little less intense but it might also be the printer I used, it's hard to say.