One very important aspect to consider: Information security.
When you tattoo a person for easy scanning, you are putting a machine readable number on them. Anyone can read that number and replicate it as a sticker to impersonate that person. This would work even at a distance, it suffices that the entire tattoo becomes visible to the attacker once.
Any biometric marker performs much better than a tattoo in this regard: The biometric marker generally can not be measured at a distance, and is generally much harder to fake without being obvious. Nevertheless, biometric markers can generally be spoofed in the same way as a tattoo. Think for example finger prints: Any person leaves their fingerprints everywhere, so it's easy for an attacker to covertly obtain the whole set of their targets fingerprints. After that it's a simple matter of creating a silicon cover for their own finger to fool any fingerprint sensor.
So, if you want real security, you must use chips. The difference is that a chip is an active component; it can perform cryptographic computations by itself. This allows the chip to
Tell the scanner its identity, and to
actually prove to the scanner that it's really that chip,
without disclosing the secret key that's used for that proof.
If the chip is built well, it cannot be copied without really, really expensive equipment, destroying the original in the process. As such, the greatest danger of using chips is, that someone's chip is taken away by force to be used by the attacker. Think: Kill the guards, cut out their chips, and enter whatever they were guarding. Impersonation is not impossible, but it becomes damn hard to cover up.