I use both snaps and velcro on mine, because it can and have been accidently bumped/slipped/ripped off and you want the armor secured. Specifically, the shoulders, use both. Snaps alone leave gaps and velcro, by itself will come off all the time.
You want to also add snaps to your knees, cod, and back plate to your jumpsuit. They will come down after a little time. Or if you don't mind pulling them up ever so often.