It seems to me that S/MIME has a better chance for wide adoption due to it being built in to many email clients (Mail on OS X/iOS at least.) However it's still up to the user to get a certificate, which is a barrier. But I'm curious what others think, as I haven't seen S/MIME brought up in the recent encrypted email threads.
The problem with S/MIME is that it is strictly hierarchical. That already hasn't worked well for TLS.
The aim of email encryption is in a large part to prevent government-level parties from reading the emails. It doesn't really make sense to then go back to a system controlled by the very same parties.
OpenPGPs web-of-trust model seems more appropriate.
But both approaches share a significant number of problems, so...
