Mirroring vs. Duplicating
 

What is the difference between mirroring and duplicating?

if you mirror the geometry, it tends to put an extra line straight down the middle...which doesnt help in organic modeling, as it can cause the model to look not quite right in that area...so most people just duplicate over and merge down the center...with mechanical modeling though, either way works fine.

oh, mirror geometry automatically merges the pieces together, while duplicating with - scale doesnt.