Luke,
I am new to these and the OB6, but the main take is that the sound editor is standalone, so for one thing it has no DAW integration. The sound editor has a whole bunch of features devoted to advanced sound design; I'm not a great designer but I've used the morpher/patch maker to create some interesting hybrid sounds kind of randomly that actually sound decent. My problem now is where to store those, and the editor (I think) does a bit more in terms of management of banks/programs.
It was only around US$30 to buy as a bundle, so to me it was worth it. I haven't played as much with PlugSE yet as I integrated initially with Bitwig, but I plan to (and hope to use bitwig's modulators to automate PlugSE params). I have not used the PlugSE effects version yet, so can't comment on that
Hope this helps; I think both products do have a few youtube videos so you can see them in action (or run limited demos)