Hello Ralph,
Thanks for your interest in S/MIME. A couple of minor corrections to your
comparison seem to be in order.
S/MIME is an attempt to graft MIME support onto underlying PEM
standards. See http://www.rsa.com/rsa/S-MIME/ for more info.
S/MIME integrates PKCS #7 and #10 message services (not PEM) into MIME.
Probably the most controversial aspect of S/MIME is its signature
format. An S/MIME signed message is a MIME multipart in which the
first part is the data to be signed, and the second part is a
complete PKCS #7 (section 10) signed message.
Although the description of this format is accurate, this format is
only documented as an option, not the primary signature format. This
option has been supplied for backward compatability to address a mixed
(S/MIME-aware and non-S/MIME aware) audience of recipients. The
primary signature format is a PKCS #7 signed message (including signed
MIME content) carried in a single body part: application/x-pkcs7-mime.