I have experience with both ... active duty 2013-2017 and currently a reservist ....I can tell they are a world apart, which one might expect. The Fleet Marine Force is a force in readiness, ready for combat and it's serious business. Haircuts, gear, vehicles, weapons, 100% ready to go and right now. It's a very professional, very macho, very sincere business.
I was shocked showing up for my first weekend drill with the reserves. Dirty uniforms, dirty boots, poor haircuts, disheveled covers ... these guys wouldn't last 5 minutes in the Fleet. It's a part time thing for them and they have part time attitudes ... I get that now. My intention is to stay active in the reserves while finishing college. I don't think I can hack the part time attitude for any longer than that.
Also ... please remember that you do not "join the Marines" .... you become a Marine. I come from a family of Marines, my father, grandfather and a half-brother are all career Marines. It's a life long thing.