With VI Brass I, you are getting the same *sound* of the brass ensembles you have in your SE, but with a plethora of new (and incredibly useful) articulations.
With Dim. Brass SE, you're getting a *new* ensemble sound to layer in, plus the humanized and divisi-friendly functionality of the dimension series.
With the BBO Brass Packs, you're getting brand new, in-situ, multi-mic libraries that - due to the multiple mics - can grant easy depth and spaciousness to the sound.
If what you're looking for is a new ensemble sound to add to your layering rather than expanding your range of available articulations, I would recommend Dim. Brass or BBO Brass packs since they are different ensembles from the ones you already have.
EDIT: I forgot to mention Fanfare Trumpets and Epic Horns. Both are bigger, separate ensembles that can be layered with your existing trumpet or horn ensembles/solos. They will *definitely* give you a bigger sound.