The new one is retina, obviously... the other is not. Old one is a 2012. I have not done any true photo editing or video editing yet... where one would most likely notice the difference... but thus far, I can't tell much difference. The old one, actually, may look better right now but I have not tweaked the new one yet.
I actually think the difference is so slight that to the human eye one would have to be an expert to really see a difference.. BUT, I'm going by the standard apps... like Pages and Numbers... and have not tossed up a truly high resolution photo or video yet.
The new one is faster, I noticed.. and that surprised me somewhat. I noticed it when I put the video of my grandson on youtube today. In normal activity I can't tell any difference. I got the 4.2Ghz with 32Gb of ram. The old one is.. hmmmm, I think a 3.2Ghz and 32Gb of ram. the difference, I'm sure, is the video card... which is still no where near as powerful as windows gaming computers... (if tricked out).