I'm not sure if I am adding useless information at this point, but when I am comparing screen sizes, I am mostly interested in what effective resolution I will be able to use.
The 27" 5K iMac is 5120 x 2880 pixels and the default effective resolution is 2560 x 1440 (218 PPI)
The 24" 4.5K iMac is 4480 x 2520 pixels and the default effective resolution is 2240 x 1260 (219 PPI)
I find both of those resolutions very usable and quite similar for fitting work on the display.
I'm a big fan of dual monitors anyway, so 2240x1260 is way more than enough screen real estate for me.
As a side note:
I do not have an iMac, but my favorite monitors are 24" 4K monitors because of how sharp the text is when scaling 2x to an effective resolution of 1920 x 1080 from the native 3840 x 2160. I much prefer this over scaling a 27" 4K monitor to other resolutions (like 2560 x 1440). The worse part about a 27" 4K monitor is that at its best 2x scaling rate of 1920 x 1080, everything looks way too big. Plus, as the monitor size increases you are losing pixels per inch and the sharpness that we come to expect nowadays.
There is a reason why Apple made 27" displays 5K instead of 4K. The fact that the iMac 24" is actually 4.5K instead of 4K is really nice, since you are getting a bit more real estate than 1920 x 1080. Even though I don't need another Mac, the fact that really nice 24" displays are hard to find - and Apple is making one for the iMac, makes me kind of want one....