In my opinion, S/MIME is, at the moment, more practical than PGP because its trust model is more clearly defined, because it's already supported by popular email clients, and because key distribution is built into the protocol.
PGP has such a loosely-defined trust model that the average user won't bother getting their key signed (or checking key fingerprints), and it becomes useless for verifying identity. The PGP concept of a "chain of trust" also starts to break down in large communities (like the world) unless there are enough individuals that spend their lives traveling from key signing party to key signing party linking together neighborhoods.
S/MIME with X.509 is more practical, because once you've proven your identity to a central organization like Thawte or CACert, your key is immediately trusted by everyone.
I like CACert right now, because it's an non-profit organization that offers keys for free, but its root is not currently distributed with most computers and web browsers. Either way, installing a root is much easier than setting up and maintaining a PGP install.
(For the super-paranoid, of course, PGP is superior because there's no central organization with the power to issue a duplicate key with your name and email address to a shady TLA.)