The benefit to preordering is that you unlock the Greek factions for free.
I can't believe they are putting the greek factions (Sparta, Athens, Epirus) in as Day 1 DLC/Preorder bonuses. It seems increadibly insulting to put some of the most iconic ancient factions from the mediterranean (not to mention ones that were free in the first game) and locking them behind a paywall. Sadly I can see them doign something similar to Parthia and the Selucids as well. I seem to recall the devs promising that they wouldn't cut content out for DLC this time around, It's nice to know how much their word is worth that they are willing to cut out some of the core factions to make a quick buck.
To add insult to injury, some of Macedon's units can't be used unless you buy the DLC.
I yearn for the days of Rome Total War/Medieval II Total War at this point. All this preorder and DLC nonsense giving access to units and factions is really irritating and blatent money grabbing, not to mention it is absolutely devestating to the amazing Total War modding community.