Both being IPS displays, it really just comes down to prettiness of retina display or real world 2560x1440 screen real estate on the 27". Even though the retina technically has more pixels, you can only scale to 1920x1200 and even that looks really small on a 15" screen.
The dots are visible on all iMac screens. The dots are invisible on all apple retina notebooks. Both the iMac and retina notebooks are good for photo editing (the color accuracy of the displays of both are reasonably accurate). So it's a matter of personal taste as to whether you prefer a huge 27" screen with visible dots or a 15" inch screen with invisible dots.
If you really like the fine detail resolution of the retina iPad then you'll like the 15" retina notebook even more.
I think the big difference would be the number of pixels per square inch on the two screen. If you want to do changes to really small areas you zoom into the pixel (on the picture). On the retina screen that pixel will be broken down into more pixels than on the big screen. Understand your camera does not have the resolution of the retina screen.
The print made from changes to a single pixel of the picture would only be visible to the most trained of eyes. Changes made to a part of that pixel using the retina screen resolution wouldn't be visible on a print of the picture.
I guess what I am saying is it really doesn't matter which screen you use to photoshop (verb, not noun) your picture if you are going to display it as a print. Or, for that matter on a high def. TV. So I would pick which ever left my eyes feeling less tired when I was finished.
All this is out the window if you have a 1 gig sensor camera that lets you zoom down to the microscopic level.
Thanks for comments. I will probably go with rMBP and then after some time I can buy seperate display which could be 4K. If I buy now an iMac I will definitely have to buy onather laptop in the future. Thus, the first combination seems more reasonable I think.
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.