Why though? For this use, isnt it more about ram? For scrolling between pictures, maybe? But that is pretty heavily indexed and optimized.
Sure disk speed matters, but for photography? What am I not thinking of?
Everything.
The SSD will be SUPERIOR... for EVERYTHING...
What he said.
Photography and AV (actually video) hits the trifecta: CPU, RAM and write-to-disk speed. Add to that, GPU. All components of a Mac or PC are involved—the better they are, the faster your edits will render. Before video editing became popular, photography was the real world benchmark task that was used to measure performance.
(all times approximate) A SATA III SSD is more than 6x faster than an HDD (an iMac doesn't have this unless the HDD is replaced). The HDD in the 2015-17 is
slower that those in the 2009–2014 to lessen problems caused by the excess heat (yes, really). The NVMe blade in a 2015 is 5x faster than a SATA III SSD. The write-to-disk speed with the NVMe blade in a 2017 is 2x faster than the blade in a 2015.
This makes WTD 60x faster in a 2017 SSD than the basic HDD version. You won't notice a huge difference if, say, writing a book in Word but photography? Absolutely.
Likewise, an i7 is much faster for computing those edits. More RAM lessens the virtual memory switchouts to your drive—64GB isn't too much especially if Photoshop is involved.
Interestingly, you can buy a used 2015 Fusion iMac, replace the NVMe blade with a 3x4 such as the 970 EO, max out the RAM and toss that HDD (or replace it with a SATA III SSD for more storage) for not that much money. That machine will perform nearly as well as a 2017 iMac of the same specs. There are many threads on how to do this. The HDD only version of the 21.5 iMac might not have the bus connector for the NVMe blade SSD (many do but no one knows till the machine is opened up). All 27" iMacs have both busses no matter how they are equipped.
I don't know the pricing on an iMac Pro where you live but it is perfect right out of the box. So is the 2017 iMac i7 with 32G (or more) RAM and the Radeon 580 GPU. I like a 1TB SSD since it lessens the amount of housekeeping you must do. Active work files should always on the boot drive while externals are used for inactive files—bring them into your boot drive when working on them.