1ml = 1cc, those are a measure of the volume...that is, how much liquid is in the amp.
mg = saturation content of the liquid...that is, how much of the drug is suspended within the given amount of liquid, ml's or cc's. So if you have deca 300 for example, by tokkyo labs, then you have 300mg's of deca in every 1ml or lcc shot that you do. The more mg's in any one given ml or cc the better, less voluminous injections.