heres a simple rule:

use smigrate if you dont care about users  or permissions and just want data, or need to move a WSS subsite to another place.
use stsadm if you want to create an IMAGE of the site  - retaining EVERYTHING (except of course wppacks that are custom that the site uses)


the ONLY caveat to using stsadm is that you cannot backup  a "subsite" of a WSS site - for that you have to use smigrate.