1440p is the native resolution of the non-retina 27" iMac. For the Retina iMac it is 2880p, so 1440p is half of that and there is some slight blurriness since you're not running at native resolution, as is always the case with LCD screens. So, while a 2880p resolution on a 5K iMac will, of course, look better than 1440p on a non-retina one, when they are both at 1440p, the non-retina one will have a sharper image. Still, 1440p does look well on 5K because it is exactly 2x smaller.
OK, thanks
I though that it would have the exact same appearance on the Retina because it has exactly 4x the count of pixels of the non Retina, but it seems more complicated than that.