I'm on a 2017 MBA Air. Currently on Monterey. And frankly, for what I use it for (docs and pdf creation, some minor video and pic editing, listening to music, email, internet - including streaming, iCloud communication between it and my iPhone), I've not seen significant differences / improvements between the original Catalina and now.
That said - as you noted - Monterey will be the last OS to work on the it, and I'd be inclined to believe Apple would be more likely to maintain some security and OS apps updates on it than they would with Catalina. So maybe if the plan is to hold onto the device for awhile yet, that might be a consideration?
Interesting consideration as noted by StevenGeorge, as far as size of the OS is concerned. Not sure of the size difference between Catalina and Monterey, but an installation does overwrite the other, too - so not using up the space of Catalina plus Monterey. Still, if your SSD is already borderline full, that might be a thought.