Should I update to El Capitan?
Grey, like everything in life you have choices. Follow your heart.

The short answer is YES.

"ElCap" (my short name because I think the name is too long and a bit weird) is like what we did with Leopard 10.5 to Snow Leopard 10.6. Leopard was about features. Snow Leopard was about behind the scenes enhancements.

Elcap is a foundational performance and efficiency upgrade unlike Yosemite which was the radical looks, feel, feature upgrade. That the names between Yosemite (the national park) and El Capitan (the climbing peak inside the park) are related is intentional (like Leopard and Snow Leopard) selected to infer that ElCap is all about the details and not the big picture. The majority of the work is not visible representing a tightening of all the core architectures.

I've worked with every beta we've produced. I even have ElCap running on a variety of platforms right down to a 2008 iMac. It's no speed daemon on that unit but it's quite usable with only 4GB of RAM. It's a testament to Apple hardware and software that our software upgrades tend to strive to minimize performance hits and actually, in some cases, increase performance.

Maybe if you were running an older unit I'd be less certain but in your case, for all the tightening of code and security and cloud refinements, ElCap makes sense with less risk then most upgrades I've seen.

I could go into all the changes underneath the code like efforts to make the OS more secure by making it 'rootless' but if performance is your concern I think you can be at peace with the upgrade.

Hope that helps.