biotin
I would pick the 5000mcg, which is 5000 micrograms. There is 1000mcg (or micrograms) in a milligram (mg) So your 5000mcg is a also 5mg (milligrams) tablet or capsule.
I take 5 mg (or 5000mcg, it depends on what the manufacturer puts on the label) every day and have, for going on 4 years. You need at least this much to kickstart the growth of your hair. The 1000mcg is a very weak amount of biotin and probably won't do much for your hair growth.
Hope this helps!