There's no privacy in either case. Every single email passes in clear text through some ISP who may very well be storing copies of them. If you want privacy, you use PGP.
"Usability" is ill-defined. GPG seems to have pretty good "usability" from mutt (which auto signs outgoing email and optionally encrypts with a keypress). Maybe your MUA is broken or you forgot to RTFM.
Usability (as I've employed it) is when someone non-technical can derive benefit from something beyond their understanding. Rest assured I've read every line of the GPG man page and then some.
