Hideo Kojima has described Metal Gear Solid V: Ground Zeroes as a tutorial to the upcoming The Phantom Pain.
GZ is more like a tutorial, so fairly small open world, but TPP has much bigger field & changes time/weather giving more tension to players.
There’s no doubt that Metal Gear Solid V: Ground Zeroes is a great game, but the asking price has annoyed some gamers due to its short length.
Nevertheless, The Phantom Pain will be a legitimate full game. If you’ve played Ground Zeroes already, you will already know how great the actual gameplay is despite its short length.