The current content for the Collector's Edition is locked in at this point, but there are some details about the Field Manual version in the CE that people may not be aware of. While we announced the Field Manual can be purchased separately, there are some bonuses to the CE version.
A collection of concept artwork from Legacy of the Void (16 pages)
The "Spear of Adun" page folds out to fill 3 pages
There is a poster packed into the back of the book
So while the Field Manual is already cool enough, the CE version has some really nice extra goodies.
Source (Psione, SC2 Community Manager):
https://www.reddit.com/r/starcraft/comments/3nqtjs/new_zealot_response_sound_in_30_we_embrace_the/cvql94t
CE Purifier Colossus skin: