One is a password, the other is an unlock phrase for a securely generated key. Password based keys need to go through PKDFs and have limited entropy. A "PIN" i

One is a password, the other is an unlock phrase for a securely generated key. Password based keys need to go through PKDFs and have limited entropy. A "PIN" in the TPM sense is alphanumeric and will be entered the same way the user enters a passphrase, except the secret that actually protects the disk isn't derived from that passphrase. It's much harder to brute force a TPM PIN than it is to brute force your average PKDF function. The brute force complexity grows to encompass the full key space rather than the key space of a key derived from memorable alphanumeric text. With a proper passphrase this probably won't matter much in terms of feasibility, but a lot of users pick terrible passwords that are easy to brute force with a whole bunch of servers, while with a TPM chip an attacker would need to choose another method (i.e. attacking the TPM itself).