Mojave better than High Sierra 10.13-6? Is it worth the effort?

Each progressive OS build either corrects or adds things. From security issues to a new feature.

But with each release you run the risk of the footprint of the running code (in memory) is greater than what you have for RAM which then pushes into Virtual Ram space within your drives storage space. This in turn pushes you to a larger drive and/or go with faster drive technology (HDD to SSD).

The last factor is the API’s the Applications you want to run need. As Apple loves to push things you might find the App you want requires a newer OS. Even still some newer hardware features won’t be present in your older system.

So strictly answering your question: High Sierra’s footprint is smaller than Mojave.

Now to really mess you up! You aren’t limited to what Apple forces you to do giving up your system as they don’t support it with the newer OS’s. Here we can cheat! Using what is called a Shim Service which translates the newer API calls to hopefully compatible older API calls allowing you to install a newer OS on your system and run newer Apps which need newer API support. There are a few most are free or a small fee.

But there is a warning here! You need to build a bootable drive with the OS installer/s and be aggressive with your backups as any security or maintenance update could mess you up so you need to rebuild everything. And if the developer stops supporting the Shim service you might be at the end of the road. Depending on your needs this maybe worth the risks!