By stronger, I assume you mean "more concentrated."

Each of the units you are asking about are masses - usually used to indicate the amount of medication delivered per dose - and are generally seen written as "mg per tablet," or "mcg per ml," etc. The "g" in each refers to "grams", and since this is metric, we only have to decipher the prefixes to know the units (for a list of S.I. prefixes, click here). The first one is straightforward, "m" is the prefix for "milli-" meaning "one-thousandth of", so 1 mg is "one milligram", or "one thousandth of a gram." The second prefix, "mc", is a bit trickier, since it is not an S.I. prefix. "mc" is a medical abbreviation for "micro-" meaning "one millionth of." The S.I. prefix for "micro-" is "µ", which doesn't appear on most pharmacists' keyboards; hence the usage of "mc" instead. So, 1 mcg is the same as 1 µg - "one microgram" or "one millionth of a gram."

1 g = 1000 mg
1 mg = 1000 mcg (µg)

So, a dosage in mg's will have a thousand times more medicine than the same value in mcg's.