Foxfifth offered some good points.
I have a late 2015 iMac with 8 GB memory and an internal SSD. It has Monterey and runs fast and smoothly.
Advantages of Monterey: you get the latest security updates, and can use the latest browsers. If you do purchase or install third party software, Monterey is the latest version and some software will not run under El Capitan, it is too old. We were using an older iMac with EL Capitan (10.11) and I was seeing that some web sites were starting to not display properly. Also the version of Apple Mail under El Capitan was not displaying some emails properly.
Another advantage is that your internal SSD will be reconfigured for APFS under Monterey when/if you upgrade, and APFS is optimized for SSDs.
Disadvantage, as pointed out previously, is that older 32-bit software will not run and will need to be replaced with the newer versions. Some older printers and scanners might not run. However we have a Canon printer/scanner we got in 2011 and an Epson printer/scanner obtained in 2015 and both have worked fine under El Capitan through and including Monterey.
Generally, I tend to lean toward installing the latest MacOS that will is compatible (as long as you have an SSD; with an internal HDD, going beyond Sierra or High Sierra might not be advisable because HDDs don't run well under APFS). Monterey is now on 12.5 which means many bug fixes and tweaks to improve performance have already been made, it's a mature MacOS.
