Is it preferable to buy two 8GB or one 16GB for iMac 27" 2017 memory upgrade?
Solution 1:
Your model, like most Macs, supports memory interleaving, which means that paired memory may work better than single modules.
However, Apple's own guidelines for installing memory in iMacs make no mention of this, and the real-world effect may be minimal, (as mentioned in comments)
Solution 2:
After reading the posts that @bmike posted I made up my mind and I'll opt for one module of 16GB and I'll have one memory slot still available just in case.