The 21.5 “iMac that Apple currently sells uses 8th generation CPUs as opposed to the 10th generation CPUs in today’s 27” iMac. 21.5 “was actually released in March 2019 and has not been updated since other than the storage options. From my own personal experience (I have basically used every iMac from the original G3 to a 2011 27”) I would rather have a larger screen. I’ve found that compared to the 21.5 “iMac, the 27” iMac just feels better for having more things open.
Edit: This is most relevant if you choose to get one of the iMac models currently on offer.