ParticleSystem.Stop or particleEmitter.enabled = false;

This is really not something that could pull back your performance. Not worth to invest energy into things like this. ParticleSystem.Stop() will also look for child particlesystems and stop them as well so it is comfy to use in a situation when you would like to stop a hiearchy of particleemitters.

You could consider readability, length or anything, but beleive me in terms of performance it is negligible