I have read about this encryption feature for OS X but want to know how secure it is compared to PGP.
Here is the official documentation for enterprise about Apple's full disk encryption solution FileVault 2.
FileVault 2 uses the user's login password as the encryption pass phrase. It uses the AES-XTS mode of AES with 128 bit blocks and a 256 bit key to encrypt the disk.
The problem there is that the user's password used to encrypt the key are generally pretty weak unless the policy which required long and complex password is enforces.
XTS mode is susceptible to data manipulation and tampering. It is also susceptible to traffic analysis, replay and randomisation attacks on sectors and 16-byte blocks. As a given sector is rewritten, attackers can collect fine-grained (16 byte) ciphertexts, which can be used for analysis or replay attacks (at a 16 byte granularity). It would be possible to define sector-wide block ciphers, unfortunately with degraded performance.
When you are speaking about "PGP-Disk Encryption", I guess your are speaking about TrueCrypt aka VeraCrypt (forked now). VeraCrypt uses the XTS mode of operation for AES which presents the same vulnerabilities as above. It supports also other combinations of cascaded algorithms such as are available: AES-Twofish, AES-Twofish-Serpent, Serpent-AES, Serpent-Twofish-AES and Twofish-Serpent. This cascading combination of different symmetrical encryption algorithm presents a better security since if one of them is affected by an attack you'll still have the break the other layer.