i use plugs primarilly.. but put muffs over top when at an energetic range.. or when firing shoulder cannons..
( PS.. I work in an audio job.. and I can tell you that plugs are (can be) MUCH more effective than muffs ).
that said.. you can't compair a CHEAP, poor fitting low NRR plug to a high quality, high NRR, well fitting muff.
in the end.. CYA and use both...
you also have to lok at noise makeup.. freq involved and the actual NRR of the device you are using.. the class of noise.. also onset speed.. etc..
duration of exposure and exposure type, constant or intermittant..e tc.
lotsa variables other than looking at a number and checking fit.