Substance is a re-edition of Metal Gear Solid 2: Sons of Liberty with many new features and functions. Simily to what VR Missions did to Metal Gear Solid, Substance adds exactly that (substance) to the game. And it includes complete game, i.e. it is not just an add-on. But it also does a lot more...
Additionally to the new VR Missions (i.e. Raidens training), there are many more bonus games, such ass Metal Gear Skateboarding, Metal Gear Photoshoot, new Missions, the option to play either chapter of the game with either character (so if you hate Raiden, you can play the entire game as Snake, Boss Battle Mode, viewing the games´ cutscenes with swapped characters, including Men in Black, Housewives, etc. The new missions are fun to play even if they do not add much to the story, but should provide further challenges to those of us who can`t get enough of Metal Gear. The VR-Missions are graded from easy to hard and should provide many more hours of play value.
Official Metal Gear Games List:
Metal Gear - Metal Gear 2: Solid Snake - (Metal Gear: Ghost Babel)
Metal Gear Solid (see also Twin Snakes) - Metal Gear Solid 2: Sons of Liberty (Substance) - Metal Gear Solid 3: Snake Eater