Square Enix has revealed the Final Fantasy XV pre-order bonuses for the digital versions of the Playstation 4 and Xbox One through a couple of promotional images. The bonuses are the same as far as in-game bonuses go but there are some other platform specific goodies.
An Xbox One digital pre-order will net you the "Masamune DLC Weapon", which you may recognize as Sephiroth's sword in Final Fantasy VII. It will also get you the Angler DLC Kit which is described as thus, "Take this handy tackle box to the fishing holes around Eos, snag yourself a whopper, and claim the title of Ace Angler." It's unclear to me whether that adds a fishing mini-game to Final Fantasy XV that otherwise wouldn't be present or if it's simply an expansion for an already existing minigame. You will also receive the "16 Bit Buddies DLC Pack" which consists of stickers of the the main cast of Final Fantasy XV in a 16 bit rendition that you can use to decorate your in-game car. The Xbox exclusive pieces are all avatar items. You will get a Carbuncle pet and a male and female version of the Noctis costume for your Xbox Live avatar.
A PS4 digital pre-order will net you the Masamune DLC Weapon, Angler DLC Kit, 16 Bit Buddies DLC Pack and on the exclusive side the Big Bang Dynamic Theme featuring Yoshitaka Amano's art. These are the bonuses for pre-ordering the digital versions of the game through Xbox Live and PSN respectively.
There are still many different editions of the game and different bonuses for pre-ordering at different retail outlets so get a spreadsheet ready to decide which version to buy before the game releases on September 30th of 2016. You can currently play the Final Fantasy XV Platinum Demo on both PS4 and Xbox One to get a taste.