Ear plugs or ear defenders - which is best?

There is no 'best' and provided both have been assessed as being suitable for the noise risk (done as part of the noise assessment) then both are as good as each other. 

'Tis true that ear defenders (ear muffs) can be more powerful than plugs but more powerful is not synonymous with 'best' and you usually do not want the most powerful protection on the market. See the section on hearing protection for info. on why and how to choose protection.

As a recommendation, any workplace with a noise risk should have at variety of styles available for people, including at least one set of plugs and one set of muffs/defenders. Some people hate plugs, some people hate the squeeze of defenders, so have both available and they can choose which they prefer.