I would ask myself the following questions:

1. Does the computer fulfill my needs? If not, will it if I upgrade by adding RAM and/or a higher capacity hard drive?

2. Will the computer run the latest OS? It's hard to judge if it will run future OS's but the chances are that, sometime in the future, it will not. Both of yours will run the latest version.

3. How much am I willing to spend?

4. Is the computer listed as Obsolete or Vintage by Apple? If the former, Apple no longer will repair it nor supply parts. You can check with MacTracker. (For the record, your iMac is listed as Vintage and your MB Air as Supported. Vintage machines will sooner or later, probably sooner, become Obsolete.)

5. Regardless of practicality or price, would I get some pleasure from having the latest and greatest? After all, you only live once.

I was very happy with my early 2008 Mac Pro, but its maximum OS was 10.7. I was running 10.6.8 but that OS was not supported anymore.

Jon

macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365