If you do keep your original version of Legato, just make sure you use leaf shell to build your systems. The danger is that you might revert to using an older version of the toolchain if you don’t uninstall it from your dev machine.
sudo rm -rf /opt/swi will remove any old toolchains from your environment and save you the headache down the road.
stable versions have the officially released version of legato + the toolchain and firmware. dev versions have an updated legato version but keep the toolchain and the firmware at the last stable version. It depends if you want the new features or if you want to wait for the product teams to test and release a new version of legato + the firmware. All versions are tested by the Legato team on the latest version of the firmware.