It is very much better, the Commendation Medal would be awarded for heroism and/or meritorious service, the Meritorious Medal would be awarded for outstanding achievement or service to the United States.
Any Marine who does 2 years of service with no demerits (personal or professional downchecks) is eligible for the Good Conduct Medal, so yes either one of these is VERY MUCH BETTER.
They both designate OUTSTANDING performance on the part of that Marine.
Unless it's changed, in the intervening 45 years since I was in boot camp, it is 3 years for a Good Conduct medal. (something about Marines going 4 years without getting in trouble is a bridge too far)
"(personal or professional downchecks)" what the heck is a down check?
How about no busts, NJP, Courts Marshal.