At the moment I' m using a 2019 27" iMac (full specs / Intel). Works fine/fast. Specifically for photo editing (and printing), I' m considering a new 27" 1440p Eizo screen.
I'm assuming that you've done your research and have a good reason for considering a
1440p Eizo. E.g. because you have
specific, specialised needs for colour-accuracy/gamut or calibration that the Eizo meets and the (otherwise superb) 5k iMac display doesn't. I appreciate that there
are scenarios, esp. in photo editing, where colour accuracy/gamut and meeting reference standards are more important than resolution (and the iMac's P3 gamut is more video- than photo- oriented) - but for everything else it's going to feel like a hefty downgrade from the 5k iMac display.
The other "advantage" of 1440p is that you can match the default UI size of the iMac without scaling. With 4k@27" you have a three-way choice between "looks like 1080p" mode - which gives you rather large UI elements (not necessarily a problem if you hide the menu bar & dock or work in full screen mode and choose the appropriate zoom); "looks like 1440p" mode - which matches the iMac UI size at the expense of fractional scaling (which looks perfectly fine for general use IMHO - opinion seems divided - but will be an issue if you're doing pixel-accurate graphics) or finally "3840x2160" mode where the system UI is ridiculously small (marginally usable -
may be OK if your app has it's own 'large icons' mode and your eyeballs are 20 years younger than mine). The thing to realise though is that *all* of these options display far more detail than you'd get on an actual 1440p display - and you can switch between them in a jiffy to suit what you're doing. An actual 1440p display is
awful at anything other than 1440p - I think that 4k gives you far more flexibility. (That said, a few years ago, 1440p was the bee's knees - although I think MacOS and other software are now increasingly optimised for "retina" which includes 27" 4k displays at desktop viewing distances).
I've just switched from a
2017 higher-end iMac to a Mac Studio - I'd say that, in hand-wavy terms, it's about twice as fast and much quieter under load - so a 2019 iMac is going to be a closer-run thing - and you say you have no performance problems -so for you it
almost comes down to desk space.
In terms of displays - for general contrast, crispness, "pop" and matching the native MacOS UI size, it's hard to beat the 5k iMac screen so anything other than a Studio Display is going to be, objectively speaking, sub-optimal. However, there are other considerations that might outweigh "optimum" image quality - I've gone for a 3840x2560 (3:2, 28.3") display because I much prefer that aspect ratio and, in that format, actually prefer the larger UI size in "looks like 1920x1280" mode - plus, I can get 2-3 of those for ridiculous real estate, and still have change from the price of a Studio Display. In your case I guess it would be to get some colour gamut/calibration not offered by the Apple display - but going all the way down to 1440p is going to be a shock to the system after using an iMac.
If you have the desk space I'd stick with the iMac for now - use dual displays if you have a well-defined need for something like the Eizo - and wait for the next generation of Mac Studio (the next 18 months will just fly by...) or even see if the rumoured M2 Mac Mini would meet your needs. If you're sufficiently cash-insensitive to be considering a Mac Studio I'd first look to see if there are any 4k displays that meet your photo editing needs.
