It all depends on what you are doing on your Mac. If you are not looking to run the latest releases of software then sticking with Snow Leopard should be fine.
Consider upgrading if you use online banking or if you need to run the latest software. Snow Leopard is no longer supported by Steam or Dropbox and the latest versions of Spotify and VLC can no longer be installed. The most up to date supported web browser is Firefox 45.9 ESR and works well.
If you are looking to upgrade go with macOS 10.12.6 Sierra as macOS 10.13.3 High Sierra can be pain depending on the Mac it is being installed upon.
El Capitan support will finish following the release of macOS 10.14 so in effect you would be going from one unsupported release to another if you were to go with OS X El Capitan.
A straight upgrade to macOS Sierra is not possible so a clean installation is the only option which is what I would recommend anyway. Before beginning I would suggest maxing out the RAM and fitting either an SSD or SSHD Solid State Hybrid Drive as you would experience a massive improvement in performance.
To make a fresh installation macOS Sierra you will need to create a USB installer
https://lifehacker.com/how-to-make-a-bootable-macos-sierra-usb-flash-drive-1786853248
Oh and before you begin carry out a Time Machine Back up.
This is Essential.