I am new to this forum;
I have a 2013 iMac and 2012 macbook Pro and still running El Capitan on both. Debating to update to High Sierra, Mojave, or stay with El Capitan. Any suggestions?
El Capitan is no longer getting security updates from Apple.
Sierra still gets those, but will likely stop when the next macOS is released in about 6 months.
High Sierra might be a good choice, and gets you down the road another couple of years.
But, Mojave is the last macOS system that will still run 32-bit apps (such as MS Office/Word/Excel 2011) some of which I still depend on.
Dark mode is also not my fav, so I don't use it (did modify it so i have a dark dock and top menubar). And, I don't experience any of the kinds of challenges that some others report. Mojave is quite stable for me, being 5 "dot" versions, and seems pretty mature at this point.
So, I would recommend going to Mojave at this point (unless you have an iMac with just a spinning hard drive, which will prove to be a bad choice for Mojave. Even a fusion drive is better, with best choice being an internal SSD.
I'm running High Sierra on my unsupported iMac 9,1 and I don't see any performance degradation compared to El Capitan. The reason for running 10.13 for me is that Safari open tabs sync is no longer supported in El Cap with devices running iOS 11 and later, but are fully supported in HS.