At some point during the development, someone (I think it was me - sorry to the developers for all the effort that caused
) had the great idea to make SE "SP1-Compliant".
This means that the bugs in YaBB 1 Gold that have been cleared in SP1 are also cleared (or N/A) in SE.
SE does, however, have a few extra features - including some mods pre-installed (which are, of course, completely optional).
To fully understand the differences, however, we suggest you download them and try them.
You can get SE at http://www.yabb.info/download.php
and you can get SP1 at http://yabb.xnull.com/downloads.php