Better to double up...
... but, if it's either / or, here is a comparison table by the Canadian equivalent of OSHA (no, I'm not Canadian, but it was the first to pop in the search):
http://www.ccohs.ca/oshanswers/prevention/ppe/ear_prot.html
Ear Plugs
Advantages:
small and easily carried
convenient to use with other personal protection equipment (can be worn with ear muffs)
more comfortable for long-term wear in hot, humid work areas
convenient for use in confined work areas
Disadvantages:
requires more time to fit
more difficult to insert and remove
require good hygiene practices
may irritate the ear canal
easily misplaced
more difficult to see and monitor usage
Ear Muffs
Advantages:
less attenuation variability among users
designed so that one size fits most head sizes
easily seen at a distance to assist in the monitoring of their use
not easily misplaced or lost
may be worn with minor ear infections
Disadvantages:
less portable and heavier
more inconvenient for use with other personal protective equipment.
more uncomfortable in hot, humid work area
more inconvenient for use in confined work areas
may interfere with the wearing of safety or prescription glasses: wearing glasses results in breaking the seal between the ear muff and the skin and results in decreased hearing protection.