Rockstar has posted an article which goes into more detail on the bonus digital content included in the Special and Collector's editions of Grand Theft Auto V. If you're yet to pre-order your copy of the game, consider purchasing one of these editions to unlock additional content, such as...
Additional Weapons (included with Special and Collector's)
Ammu-Nation will always be stocked with two bonus firearms - the .50 Caliber Pistol and the Bullpup Shotgun - that have their own unique customization options, skins and attachments, at no additional cost. You'll also have access to the Hammer, which acts as a brutal melee weapon.
Bonus Outfits, Tattoos and More (included with Special and Collector's)
Michael, Franklin and Trevor will receive a 20% discount at all stores within Los Santos and Blaine County, along with access to special items either in their personal wardrobes or from stores. Each character will also gain 1 additional outfit, article of clothing (shirt, hoodie and jacket) and a tattoo.
Custom Characters for Grand Theft Auto Online (Collector's Edition only)
You'll get Niko Bellic, along with both Claude and Misty from Grand Theft Auto III to use as parent options within the character creation system.
Side-note: GTA Online introduces a new spin on traditional character creation with the hereditary-based creation system. Select your character’s appearance by choosing your parent’s heritage, then use the gene dominance meter to dial-in which parent your appearance favors more. Additionally, you'll customize your character's daily activities - everything from family time to illegal work and sports to couch potato hours all have an impact on your character’s appearance and skill set (and you can always rank up your skill stats more via gameplay).
Special Ability Boost (included with Special and Collector's)
The Special Ability bar will generate 25% faster.
Side-note: Michael, Franklin and Trevor each have a their own unique Special Ability, which can be activated once each of them fills up the Special Ability bar based on various actions. Michael is an expert marksman - his Special Ability slows down time in a gunfight allowing him to pick off his targets with greater ease. Franklin is a skilled driver with sharp reflexes - his Special Ability lets him slow down time while driving any road-based vehicle, allowing him to quickly navigate through traffic with ease and take corners at high speeds. Trevor is a drug-addled sociopath with a short fuse - his Special Ability sends him into a rage, causing double damage to enemies while only taking half as much damage himself when hit.
Stunt Plane Trials (included with Special and Collector's)
5 thrilling and unique challenges which will zip you through mountains, deserts, oceans, and cities as you navigate through a maze of checkpoints. Completing each Trial will improve the piloting skill of your character. Completion times can be compared with friends on a leaderboard for the trails.
Unique Vehicles and Garage Property (Collector's Edition only)
The Collector's Edition features an exclusive garage to store custom vehicles, which comes stocked with the CarbonRS sports bike and the Hotknife - an open frame 1930’s style hotrod - to begin with. The high-end Khamelion electric car is specific to GTA Online.