Are you still into buying special editions? I really, truly was, for a while there. Haven't bought one in years -- and it feels great! With Forza Motorsport 5, there are two options: a limited edition ($79.99), and a day-one edition ($59.99). At least here, your money is largely going to in-game content and not random swag that will occupy shelf space you're increasingly running out of.
In terms of cars, the limited edition includes access to the 2011 Audi RS 3 Sportback, 2012 Aston Martin Vanquish, 2013 Ford M Shelby Mustang GT500, 2013 McLaren P1, and 2013 SRT Viper GTS. There's also VIP membership access, with a 2x player reward boost, exclusive multiplayer events, 1,250 in-game car tokens, and other digital goodies. Plus, even more cars: the 1965 Shelby Cobra 427 S/C, 1987 RUF CTR Yellowbird, 1991 Mazda #55 787B, 2011 Ford F150 SVT Raptor, and 2011 Bugatti Veyron Super Sport.
As for the day-one edition, which is a free upgrade for everyone who buys Forza 5 at launch, you're getting three more vehicles: the 2010 Audi TT RS Coupe, 2013 Ford Focus ST, and 2011 Lamborghini Gallardo LP570-4 Superleggera. These cars are also included in the limited edition of the game, as you probably guessed.
Forza Motorsport 5 Limited Edition [Xbox Wire]