I returned my iMac 5k for a 27" late 2013 me088.
I think it was waste of money since i only use it to browse internet, stream video and store pics, listen to music, wright documents a few times on a year and print it.
The 2013 27" iMac have quad hd, a good screen, better than thunderbolt screen.
Fan sound is the same on the 5k and late 2013, sata 7200rpm is slower of course but not so slow that i notice it, use 45 sec to boot up vs 20-22 on 5k.
The late 2013 open programs fast also.
I miss the 5k a bit because of the screen but i can't use so mutch money just because of that, the 2013 still have quad hd, wore if it only had full hd.
Side by side comparison they almost look the same if you stand a meter from the screen, a regular guy on the street could not see witch is 5k or 2.5k i think, but when you are close up you easy see that 5k is more sharp
2013 vs 2014 retina imac is a hard choice, not easy to choose between them at all.