Encrypt with Public key or passphrase (PGP)

I have just purchased PGP WDE Desktop 10.1.2 for Mac and have encrypted my SSD. Now I want to encrypt and external HDD (And test PGP vs TrueCrypt). My concern is what is the benifit of using a Public key/private key pair vs a passphrase (PGP lets you use either to encrypt you HDD. Is it ment to allow me to send it to another person and have them access it with out sending the password insecurely? Is a passphrase better?


Also a second question:

Lately I have been seeing a lot of articles refereeing to AES-512 bit. I know that technically 128 bit is secure and even 256 bit is getting towards over kill but is 512 bit tested? Is it reliable? (I don't plan on using it just curious as I though AES only supported 256 bit key length at maximum).


Thanks for any help