Size - Obviously, in most cases, a woman's nipple size is more generous than a man's, allowing more space to insert a piece of jewelry. If a man's nipple is extremely small and tight to the chest surface, it's possible that it can't safely be pierced at all. It's important that nipple piercings do not puncture breast tissue. That could lead to a nasty infection known as mastitis, which can actually be life threatening.
Functionality - A woman who has or may have a child and wishes to breastfeed, a safely done nipple piercing does not hinder the ability to nurse. The jewelry, however, should be removed and it's very important to make sure that the fistula is clean before doing so. Any bacteria, infection, or even the jewelry itself can pose a danger to your nursing baby.
Arousal - One of the primary reasons for getting nipple piercings is to increase arousal. Not everyone enjoys nipple stimulation, but if you do then a piercing may increase that enjoyment. But it's also possible that it could actually decrease stimulation and there's no test that can be done to know beforehand which outcome you will get. Only you can decide if it's worth the risk.
Starter jewelry is usually a 16-14 gauge straight barbell, CBR, or circular barbell. Nipple shields are designed to work with straight barbells and are for healed piercings only. Healing takes a minimum of 6 months.