Overture SE is very similar to the full version of Overture. However, SE doesn\'t support lyrics, guitar frames and guitar tablature,though it does include a TAB clef and staff (maybe the developer overlooked those). Also, it doesn\'t offer control over the various \"libraries\" (e.g., fonts, chord symbols, guitar frames, drum map, expression symbols). These are the main differences, as far as I can tell.
Grab the Overture demo from here and give it test drive.
BTW, the Garritan team is working on something right now that will greatly simplify using Overture SE with GPO. More news to come shortly, I hope.